feat(i18n): expand catalog runtime and developer tooling

- add Intl-backed locale definitions, formatters, catalog loaders, and runtime hooks\n- compile hashed production catalogs from package-scoped catalog sources\n- drive Lingui extraction and compilation directly from project JSON config\n- redesign the devtool with localized controls, draggable persistence, settings, navigation, copy actions, ANSI output, and fullscreen support\n- extend repository APIs, CLI coverage, documentation, and runtime tests

packages/i18n/src/build/catalog-sources.test.ts

@@ -0,0 +1,90 @@
+import { mkdir, mkdtemp, rm, writeFile } from "node:fs/promises"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+
+import { afterEach, describe, expect, it } from "vitest"
+
+import type { I18nProjectConfig } from "../config/lingui-config"
+import {
+  loadProjectCatalogs,
+  resolveCatalogSourceFilenames,
+} from "./catalog-sources"
+
+const temporaryDirectories: string[] = []
+
+afterEach(async () => {
+  await Promise.all(
+    temporaryDirectories
+      .splice(0)
+      .map((directory) => rm(directory, { force: true, recursive: true }))
+  )
+})
+
+async function createProject() {
+  const projectRoot = await mkdtemp(join(tmpdir(), "workspace-i18n-build-"))
+  const project: I18nProjectConfig = {
+    catalogPath: "app/{locale}/messages",
+    catalogSources: ["./dependency/{locale}.js"],
+    include: ["src"],
+    locales: ["en", "zh-Hans"],
+    sourceLocale: "en",
+  }
+
+  temporaryDirectories.push(projectRoot)
+
+  for (const locale of project.locales) {
+    const dependencyDirectory = join(projectRoot, "dependency")
+    const applicationDirectory = join(projectRoot, "app", locale)
+
+    await mkdir(dependencyDirectory, { recursive: true })
+    await mkdir(applicationDirectory, { recursive: true })
+    await writeFile(
+      join(dependencyDirectory, `${locale}.js`),
+      `export const messages = ${JSON.stringify({
+        dependency: `${locale}: dependency`,
+        overridden: `${locale}: dependency`,
+      })}\n`
+    )
+    await writeFile(
+      join(applicationDirectory, "messages.js"),
+      `export const messages = ${JSON.stringify({
+        application: `${locale}: application`,
+        overridden: `${locale}: application`,
+      })}\n`
+    )
+  }
+
+  return { project, projectRoot }
+}
+
+describe("production catalog sources", () => {
+  it("resolves package sources before the application catalog", async () => {
+    const { project, projectRoot } = await createProject()
+    const filenames = await resolveCatalogSourceFilenames(
+      project,
+      projectRoot,
+      "en"
+    )
+
+    expect(filenames).toEqual([
+      join(projectRoot, "dependency", "en.js"),
+      join(projectRoot, "app", "en", "messages.js"),
+    ])
+  })
+
+  it("merges every locale and lets the application override dependencies", async () => {
+    const { project, projectRoot } = await createProject()
+    const { catalogs } = await loadProjectCatalogs(project, projectRoot)
+
+    expect(catalogs.en).toEqual({
+      application: "en: application",
+      dependency: "en: dependency",
+      overridden: "en: application",
+    })
+    expect(catalogs["zh-Hans"]).toEqual({
+      application: "zh-Hans: application",
+      dependency: "zh-Hans: dependency",
+      overridden: "zh-Hans: application",
+    })
+  })
+})

packages/i18n/src/build/catalog-sources.ts

@@ -0,0 +1,125 @@
+import { access, stat } from "node:fs/promises"
+import { createRequire } from "node:module"
+import { isAbsolute, resolve } from "node:path"
+import { pathToFileURL } from "node:url"
+
+import type { I18nProjectConfig } from "../config/lingui-config.ts"
+import { mergeMessageCatalogs } from "../runtime/catalogs.ts"
+import type { MessageCatalog, MessageCatalogs } from "../runtime/types.ts"
+
+function resolveLocaleTemplate(template: string, locale: string) {
+  return template.replaceAll("{locale}", locale)
+}
+
+async function fileExists(filename: string) {
+  try {
+    await access(filename)
+    return true
+  } catch {
+    return false
+  }
+}
+
+export async function resolveCompiledCatalogFilename(
+  project: I18nProjectConfig,
+  projectRoot: string,
+  locale: string
+) {
+  const pathname = resolveLocaleTemplate(project.catalogPath, locale)
+  const baseFilename = isAbsolute(pathname)
+    ? pathname
+    : resolve(projectRoot, pathname)
+  const candidates = [baseFilename, `${baseFilename}.ts`, `${baseFilename}.js`]
+
+  for (const candidate of candidates) {
+    if (await fileExists(candidate)) {
+      return candidate
+    }
+  }
+
+  throw new Error(
+    `Compiled i18n catalog not found for ${locale}: ${baseFilename}.{ts,js}. Run the i18n compile command before building.`
+  )
+}
+
+export function resolveCatalogSourceFilename(
+  source: string,
+  projectRoot: string
+) {
+  if (isAbsolute(source)) {
+    return source
+  }
+
+  if (source.startsWith(".")) {
+    return resolve(projectRoot, source)
+  }
+
+  return createRequire(resolve(projectRoot, "package.json")).resolve(source)
+}
+
+export async function resolveCatalogSourceFilenames(
+  project: I18nProjectConfig,
+  projectRoot: string,
+  locale: string
+) {
+  const dependencies = (project.catalogSources ?? []).map((source) =>
+    resolveCatalogSourceFilename(
+      resolveLocaleTemplate(source, locale),
+      projectRoot
+    )
+  )
+  const applicationCatalog = await resolveCompiledCatalogFilename(
+    project,
+    projectRoot,
+    locale
+  )
+
+  return [...dependencies, applicationCatalog]
+}
+
+async function importMessageCatalog(filename: string) {
+  const fileStats = await stat(filename)
+  const module = (await import(
+    `${pathToFileURL(filename).href}?workspace-i18n=${fileStats.mtimeMs}`
+  )) as { messages?: unknown }
+
+  if (
+    !module.messages ||
+    typeof module.messages !== "object" ||
+    Array.isArray(module.messages)
+  ) {
+    throw new Error(`Catalog module must export a messages object: ${filename}`)
+  }
+
+  return module.messages as MessageCatalog
+}
+
+export async function loadProjectCatalogs(
+  project: I18nProjectConfig,
+  projectRoot: string
+) {
+  const catalogs: MessageCatalogs = {}
+  const sourcesByLocale: Record<string, readonly string[]> = {}
+
+  await Promise.all(
+    project.locales.map(async (locale) => {
+      const filenames = await resolveCatalogSourceFilenames(
+        project,
+        projectRoot,
+        locale
+      )
+      const sources = await Promise.all(filenames.map(importMessageCatalog))
+
+      catalogs[locale] =
+        mergeMessageCatalogs(
+          ...sources.map((messages) => ({ [locale]: messages }))
+        )[locale] ?? {}
+      sourcesByLocale[locale] = filenames
+    })
+  )
+
+  return {
+    catalogs,
+    sourcesByLocale,
+  }
+}

packages/i18n/src/build/production-catalogs.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it } from "vitest"
+
+import {
+  compactMessageCatalogs,
+  createMessageIdSchema,
+  hashMessageId,
+} from "./production-catalogs"
+
+describe("production catalogs", () => {
+  const catalogs = {
+    en: {
+      "blocks.navigation.open": "Open navigation",
+      "blocks.navigation.title": "Navigation",
+    },
+    "zh-Hans": {
+      "blocks.navigation.open": "打开导航",
+      "blocks.navigation.title": "导航",
+    },
+  }
+
+  it("creates deterministic compact IDs", () => {
+    expect(hashMessageId("blocks.navigation.open")).toBe(
+      hashMessageId("blocks.navigation.open")
+    )
+    expect(hashMessageId("blocks.navigation.open")).toHaveLength(10)
+    expect(hashMessageId("blocks.navigation.open")).not.toBe(
+      hashMessageId("blocks.navigation.title")
+    )
+  })
+
+  it("uses one schema for every locale", () => {
+    const schema = createMessageIdSchema(catalogs)
+    const compactCatalogs = compactMessageCatalogs(catalogs, schema)
+    const openId = schema.messages["blocks.navigation.open"]
+    const titleId = schema.messages["blocks.navigation.title"]
+
+    expect(Object.keys(compactCatalogs.en)).toEqual([openId, titleId])
+    expect(compactCatalogs.en[openId!]).toBe("Open navigation")
+    expect(compactCatalogs["zh-Hans"][openId!]).toBe("打开导航")
+    expect(compactCatalogs["zh-Hans"][titleId!]).toBe("导航")
+  })
+})

packages/i18n/src/build/production-catalogs.ts

@@ -0,0 +1,79 @@
+import { createHash } from "node:crypto"
+
+import type { MessageCatalog, MessageCatalogs } from "../runtime/types.ts"
+
+const MESSAGE_ID_HASH_LENGTH = 10
+
+export interface MessageIdSchema {
+  algorithm: "sha256-base64url"
+  hashLength: number
+  messages: Readonly<Record<string, string>>
+  version: 1
+}
+
+export function hashMessageId(id: string) {
+  return createHash("sha256")
+    .update(id)
+    .digest("base64url")
+    .slice(0, MESSAGE_ID_HASH_LENGTH)
+}
+
+export function createMessageIdSchema(
+  catalogs: MessageCatalogs
+): MessageIdSchema {
+  const ids = Array.from(
+    new Set(Object.values(catalogs).flatMap((catalog) => Object.keys(catalog)))
+  ).sort()
+  const messages: Record<string, string> = {}
+  const idsByHash = new Map<string, string>()
+
+  for (const id of ids) {
+    const hash = hashMessageId(id)
+    const existingId = idsByHash.get(hash)
+
+    if (existingId && existingId !== id) {
+      throw new Error(
+        `Message ID hash collision between ${JSON.stringify(existingId)} and ${JSON.stringify(id)} (${hash}).`
+      )
+    }
+
+    idsByHash.set(hash, id)
+    messages[id] = hash
+  }
+
+  return {
+    algorithm: "sha256-base64url",
+    hashLength: MESSAGE_ID_HASH_LENGTH,
+    messages,
+    version: 1,
+  }
+}
+
+export function compactMessageCatalog(
+  catalog: MessageCatalog,
+  schema: MessageIdSchema
+): MessageCatalog {
+  return Object.fromEntries(
+    Object.entries(catalog).map(([id, message]) => {
+      const compactId = schema.messages[id]
+
+      if (!compactId) {
+        throw new Error(`Message ID is missing from the build schema: ${id}`)
+      }
+
+      return [compactId, message]
+    })
+  )
+}
+
+export function compactMessageCatalogs(
+  catalogs: MessageCatalogs,
+  schema: MessageIdSchema
+): MessageCatalogs {
+  return Object.fromEntries(
+    Object.entries(catalogs).map(([locale, catalog]) => [
+      locale,
+      compactMessageCatalog(catalog, schema),
+    ])
+  )
+}

packages/i18n/src/cli/api-plugin.ts

@@ -3,12 +3,8 @@ import type { ViteDevServer } from "vite"
 
 import type { UpdateMessageInput } from "../devtool/types"
 import { CatalogService } from "./catalog-service.ts"
-import { runLinguiCommand } from "./lingui-command.ts"
-import {
-  getLinguiConfigFilename,
-  getProjectRoot,
-  readProject,
-} from "./project.ts"
+import { runProjectLinguiCommand } from "./lingui-command.ts"
+import { readProject } from "./project.ts"
 
 const API_PREFIX = "/__i18n"
 
@@ -86,6 +82,15 @@ export function configureI18nApi(
         return
       }
 
+      if (
+        request.method === "GET" &&
+        url.pathname === `${API_PREFIX}/packages`
+      ) {
+        const service = new CatalogService(projectFilename)
+        sendJson(response, 200, await service.getPackages())
+        return
+      }
+
       if (
         request.method === "PUT" &&
         url.pathname === `${API_PREFIX}/messages`
@@ -115,15 +120,7 @@ export function configureI18nApi(
           return
         }
 
-        const result = await runLinguiCommand(
-          [
-            action,
-            ...(action === "compile" ? ["--typescript"] : []),
-            "--config",
-            getLinguiConfigFilename(projectFilename),
-          ],
-          getProjectRoot(projectFilename)
-        )
+        const result = await runProjectLinguiCommand(projectFilename, action)
         sendJson(response, 200, result)
         return
       }

packages/i18n/src/cli/catalog-service.test.ts

@@ -1,12 +1,12 @@
-import { mkdtemp, readFile, rm, writeFile } from "node:fs/promises"
+import { mkdir, mkdtemp, readFile, rm, writeFile } from "node:fs/promises"
 import { join } from "node:path"
 import { tmpdir } from "node:os"
 
 import { getCatalogs } from "@lingui/cli/api"
-import { getConfig } from "@lingui/conf"
 import { afterEach, describe, expect, it } from "vitest"
 
 import { CatalogService } from "./catalog-service"
+import { loadProjectLinguiConfig } from "./project"
 
 const temporaryDirectories: string[] = []
 
@@ -23,28 +23,31 @@ describe("CatalogService", () => {
     const directory = await mkdtemp(join(tmpdir(), "workspace-i18n-"))
     temporaryDirectories.push(directory)
     const projectFilename = join(directory, "i18n.config.json")
-    const linguiConfigFilename = join(directory, "lingui.config.ts")
 
-    await writeFile(projectFilename, "{}\n")
     await writeFile(
-      linguiConfigFilename,
-      [
-        "export default {",
-        '  sourceLocale: "en",',
-        '  locales: ["en", "zh-Hans"],',
-        "  catalogs: [{",
-        '    path: "<rootDir>/locales/{locale}/messages",',
-        '    include: ["<rootDir>/src"],',
-        "  }],",
-        "}",
-        "",
-      ].join("\n")
+      join(directory, "package.json"),
+      `${JSON.stringify({ name: "test-app", private: true }, null, 2)}\n`
+    )
+    await mkdir(join(directory, "packages/ui"), { recursive: true })
+    await writeFile(
+      join(directory, "packages/ui/package.json"),
+      `${JSON.stringify({ name: "@workspace/ui", private: true }, null, 2)}\n`
+    )
+    await writeFile(
+      projectFilename,
+      `${JSON.stringify(
+        {
+          catalogPath: "locales/{locale}/messages",
+          include: ["src"],
+          locales: ["en", "zh-Hans"],
+          sourceLocale: "en",
+        },
+        null,
+        2
+      )}\n`
     )
 
-    const config = getConfig({
-      configPath: linguiConfigFilename,
-      cwd: directory,
-    })
+    const config = await loadProjectLinguiConfig(projectFilename)
     const [catalog] = await getCatalogs(config)
 
     if (!catalog) {
@@ -58,6 +61,11 @@ describe("CatalogService", () => {
         origin: [["src/dashboard.tsx", 10]],
         translation: "Dashboard",
       },
+      "ui.common.close": {
+        message: "Close",
+        origin: [["packages/ui/src/dialog.tsx", 12]],
+        translation: "Close",
+      },
     })
     await catalog.write("zh-Hans", {
       "dashboard.title": {
@@ -66,14 +74,26 @@ describe("CatalogService", () => {
         origin: [["src/dashboard.tsx", 10]],
         translation: "",
       },
+      "ui.common.close": {
+        message: "Close",
+        origin: [["packages/ui/src/dialog.tsx", 12]],
+        translation: "关闭",
+      },
     })
 
     const service = new CatalogService(projectFilename)
+    await expect(service.getPackages()).resolves.toEqual([
+      {
+        kind: "application",
+        name: "test-app",
+      },
+    ])
     const [message] = await service.getMessages("zh-Hans")
 
     expect(message).toMatchObject({
       id: "dashboard.title",
       missing: true,
+      packageName: "test-app",
       source: "Dashboard",
       translation: "",
     })
@@ -92,5 +112,47 @@ describe("CatalogService", () => {
     expect(await readFile(catalog.getFilename("zh-Hans"), "utf8")).toContain(
       'msgstr "数据看板"'
     )
+
+    const sourceMessages = await service.getMessages("en")
+    const applicationMessage = sourceMessages.find(
+      (item) => item.id === "dashboard.title"
+    )
+    const dependencyMessage = sourceMessages.find(
+      (item) => item.id === "ui.common.close"
+    )
+
+    expect(applicationMessage).toMatchObject({
+      editable: false,
+      packageName: "test-app",
+    })
+    expect(dependencyMessage).toMatchObject({
+      editable: true,
+      packageName: "@workspace/ui",
+    })
+
+    await expect(
+      service.updateMessage({
+        catalog: applicationMessage?.catalog ?? "messages",
+        id: "dashboard.title",
+        locale: "en",
+        translation: "Customized dashboard",
+      })
+    ).rejects.toThrow("must be changed in source code")
+
+    const customizedDependency = await service.updateMessage({
+      catalog: dependencyMessage?.catalog ?? "messages",
+      id: "ui.common.close",
+      locale: "en",
+      translation: "Dismiss",
+    })
+
+    expect(customizedDependency).toMatchObject({
+      editable: true,
+      source: "Close",
+      translation: "Dismiss",
+    })
+    expect(await readFile(catalog.getFilename("en"), "utf8")).toContain(
+      'msgstr "Dismiss"'
+    )
   })
 })

packages/i18n/src/cli/catalog-service.ts

@@ -1,16 +1,98 @@
-import { getCatalogs } from "@lingui/cli/api"
-import type { CatalogType, MessageType } from "@lingui/conf"
-import { getConfig } from "@lingui/conf"
+import { readFile } from "node:fs/promises"
+import { dirname, join, resolve } from "node:path"
 
-import type { DevtoolMessage, UpdateMessageInput } from "../devtool/types"
-import { getLinguiConfigFilename, getProjectRoot } from "./project.ts"
+import { getCatalogs } from "@lingui/cli/api"
+import type {
+  CatalogType,
+  LinguiConfigNormalized,
+  MessageType,
+} from "@lingui/conf"
+
+import type {
+  DevtoolMessage,
+  DevtoolMessagePackage,
+  UpdateMessageInput,
+} from "../devtool/types"
+import { getProjectRoot, loadProjectLinguiConfig } from "./project.ts"
 
 interface CatalogEntry {
   catalog: Awaited<ReturnType<typeof getCatalogs>>[number]
   key: string
 }
 
-async function getCatalogEntries(config: ReturnType<typeof getConfig>) {
+interface PackageManifest {
+  name?: unknown
+}
+
+class PackageNameResolver {
+  readonly #directoryCache = new Map<string, Promise<string | undefined>>()
+  readonly #projectRoot: string
+  readonly #projectPackageName: Promise<string>
+
+  constructor(projectRoot: string) {
+    this.#projectRoot = projectRoot
+    this.#projectPackageName = this.#findNearestPackageName(projectRoot).then(
+      (name) => name ?? "project"
+    )
+  }
+
+  async resolve(origins: MessageType["origin"] | undefined) {
+    for (const [filename] of origins ?? []) {
+      const packageName = await this.#findNearestPackageName(
+        dirname(resolve(this.#projectRoot, filename))
+      )
+
+      if (packageName) {
+        return packageName
+      }
+    }
+
+    return this.#projectPackageName
+  }
+
+  getProjectPackageName() {
+    return this.#projectPackageName
+  }
+
+  #findNearestPackageName(directory: string): Promise<string | undefined> {
+    const normalizedDirectory = resolve(directory)
+    const cached = this.#directoryCache.get(normalizedDirectory)
+
+    if (cached) {
+      return cached
+    }
+
+    const result = this.#readNearestPackageName(normalizedDirectory)
+    this.#directoryCache.set(normalizedDirectory, result)
+    return result
+  }
+
+  async #readNearestPackageName(
+    directory: string
+  ): Promise<string | undefined> {
+    try {
+      const manifest = JSON.parse(
+        await readFile(join(directory, "package.json"), "utf8")
+      ) as PackageManifest
+
+      if (typeof manifest.name === "string" && manifest.name.length > 0) {
+        return manifest.name
+      }
+    } catch {
+      // Source files may sit below directories without a package manifest.
+    }
+
+    const parent = dirname(directory)
+
+    if (parent === directory) {
+      return undefined
+    }
+
+    return this.#findNearestPackageName(parent)
+  }
+}
+
+async function getCatalogEntries(config: LinguiConfigNormalized) {
   const catalogs = await getCatalogs(config)
 
   return catalogs.map<CatalogEntry>((catalog, index) => ({
@@ -20,16 +102,20 @@ async function getCatalogEntries(config: ReturnType<typeof getConfig>) {
 }
 
 function toDevtoolMessage({
+  applicationPackageName,
   catalog,
   id,
   locale,
+  packageName,
   sourceEntry,
   sourceLocale,
   targetEntry,
 }: {
+  applicationPackageName: string
   catalog: string
   id: string
   locale: string
+  packageName: string
   sourceEntry: MessageType | undefined
   sourceLocale: string
   targetEntry: MessageType | undefined
@@ -48,37 +134,52 @@ function toDevtoolMessage({
   return {
     catalog,
     comments: targetEntry?.comments ?? sourceEntry?.comments ?? [],
+    editable: locale !== sourceLocale || packageName !== applicationPackageName,
     id,
     missing: locale !== sourceLocale && translation.length === 0,
     obsolete: Boolean(targetEntry?.obsolete ?? sourceEntry?.obsolete),
     origins: (sourceEntry?.origin ?? targetEntry?.origin ?? []).map(
       ([file, line]) => ({ file, line })
     ),
+    packageName,
     source,
+    sourceLocale,
     translation,
   }
 }
 
 export class CatalogService {
-  readonly config: ReturnType<typeof getConfig>
-  readonly catalogs: Promise<readonly CatalogEntry[]>
+  readonly context: Promise<{
+    catalogs: readonly CatalogEntry[]
+    config: Awaited<ReturnType<typeof loadProjectLinguiConfig>>
+    packageNameResolver: PackageNameResolver
+  }>
 
   constructor(projectFilename: string) {
-    this.config = getConfig({
-      configPath: getLinguiConfigFilename(projectFilename),
-      cwd: getProjectRoot(projectFilename),
-    })
-    this.catalogs = getCatalogEntries(this.config)
+    const packageNameResolver = new PackageNameResolver(
+      getProjectRoot(projectFilename)
+    )
+
+    this.context = loadProjectLinguiConfig(projectFilename).then(
+      async (config) => ({
+        catalogs: await getCatalogEntries(config),
+        config,
+        packageNameResolver,
+      })
+    )
   }
 
   async getMessages(locale: string) {
-    this.assertLocale(locale)
+    const { catalogs, config, packageNameResolver } = await this.context
+
+    this.assertLocale(config.locales, locale)
+    const applicationPackageName =
+      await packageNameResolver.getProjectPackageName()
 
-    const catalogs = await this.catalogs
     const messages = await Promise.all(
       catalogs.map(async ({ catalog, key }) => {
         const [sourceCatalog = {}, targetCatalog = {}] = await Promise.all([
-          catalog.read(this.config.sourceLocale),
+          catalog.read(config.sourceLocale),
           catalog.read(locale),
         ])
         const ids = new Set([
@@ -86,28 +187,50 @@ export class CatalogService {
           ...Object.keys(targetCatalog),
         ])
 
-        return Array.from(ids)
-          .sort((left, right) => left.localeCompare(right))
-          .map((id) =>
-            toDevtoolMessage({
-              catalog: key,
-              id,
-              locale,
-              sourceEntry: sourceCatalog[id],
-              sourceLocale: this.config.sourceLocale,
-              targetEntry: targetCatalog[id],
+        return Promise.all(
+          Array.from(ids)
+            .sort((left, right) => left.localeCompare(right))
+            .map(async (id) => {
+              const sourceEntry = sourceCatalog[id]
+              const targetEntry = targetCatalog[id]
+              const packageName = await packageNameResolver.resolve(
+                sourceEntry?.origin ?? targetEntry?.origin
+              )
+
+              return toDevtoolMessage({
+                applicationPackageName,
+                catalog: key,
+                id,
+                locale,
+                packageName,
+                sourceEntry,
+                sourceLocale: config.sourceLocale,
+                targetEntry,
+              })
             })
-          )
+        )
       })
     )
 
     return messages.flat()
   }
 
-  async updateMessage(input: UpdateMessageInput) {
-    this.assertLocale(input.locale)
+  async getPackages(): Promise<readonly DevtoolMessagePackage[]> {
+    const { packageNameResolver } = await this.context
+
+    return [
+      {
+        kind: "application",
+        name: await packageNameResolver.getProjectPackageName(),
+      },
+    ]
+  }
+
+  async updateMessage(input: UpdateMessageInput) {
+    const { catalogs, config, packageNameResolver } = await this.context
+
+    this.assertLocale(config.locales, input.locale)
 
-    const catalogs = await this.catalogs
     const entry = catalogs.find((candidate) => candidate.key === input.catalog)
 
     if (!entry) {
@@ -115,7 +238,7 @@ export class CatalogService {
     }
 
     const [sourceCatalog = {}, targetCatalog = {}] = await Promise.all([
-      entry.catalog.read(this.config.sourceLocale),
+      entry.catalog.read(config.sourceLocale),
       entry.catalog.read(input.locale),
     ])
     const sourceEntry = sourceCatalog[input.id]
@@ -125,6 +248,21 @@ export class CatalogService {
       throw new Error(`Unknown message: ${input.id}`)
     }
 
+    const packageName = await packageNameResolver.resolve(
+      sourceEntry?.origin ?? currentEntry?.origin
+    )
+    const applicationPackageName =
+      await packageNameResolver.getProjectPackageName()
+
+    if (
+      input.locale === config.sourceLocale &&
+      packageName === applicationPackageName
+    ) {
+      throw new Error(
+        "Application source messages must be changed in source code."
+      )
+    }
+
     const nextCatalog: CatalogType = {
       ...targetCatalog,
       [input.id]: {
@@ -137,17 +275,19 @@ export class CatalogService {
     await entry.catalog.write(input.locale, nextCatalog)
 
     return toDevtoolMessage({
+      applicationPackageName,
       catalog: entry.key,
       id: input.id,
       locale: input.locale,
+      packageName,
       sourceEntry,
-      sourceLocale: this.config.sourceLocale,
+      sourceLocale: config.sourceLocale,
       targetEntry: nextCatalog[input.id],
     })
   }
 
-  private assertLocale(locale: string) {
-    if (!this.config.locales.includes(locale)) {
+  private assertLocale(locales: readonly string[], locale: string) {
+    if (!locales.includes(locale)) {
       throw new Error(`Unknown locale: ${locale}`)
     }
   }

packages/i18n/src/cli/index.ts

@@ -3,13 +3,9 @@
 import { cac } from "cac"
 
 import { startDevtool } from "./dev-server"
-import { runLinguiCommand } from "./lingui-command"
+import { runProjectLinguiCommand } from "./lingui-command"
 import { addLocale } from "./new-locale"
-import {
-  getLinguiConfigFilename,
-  getProjectRoot,
-  resolveProjectFilename,
-} from "./project"
+import { resolveProjectFilename } from "./project"
 
 const cli = cac("workspace-i18n")
 
@@ -18,15 +14,7 @@ async function runProjectCommand(
   projectOption?: string
 ) {
   const projectFilename = await resolveProjectFilename(projectOption)
-  const result = await runLinguiCommand(
-    [
-      command,
-      ...(command === "compile" ? ["--typescript"] : []),
-      "--config",
-      getLinguiConfigFilename(projectFilename),
-    ],
-    getProjectRoot(projectFilename)
-  )
+  const result = await runProjectLinguiCommand(projectFilename, command)
 
   if (result.output.trim()) {
     process.stdout.write(result.output)

packages/i18n/src/cli/lingui-command.test.ts

@@ -0,0 +1,69 @@
+import { mkdtemp, readFile, rm, stat, writeFile } from "node:fs/promises"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+
+import { getCatalogs } from "@lingui/cli/api"
+import { afterEach, describe, expect, it } from "vitest"
+
+import { runProjectLinguiCommand } from "./lingui-command"
+import { loadProjectLinguiConfig } from "./project"
+
+const temporaryDirectories: string[] = []
+
+afterEach(async () => {
+  await Promise.all(
+    temporaryDirectories
+      .splice(0)
+      .map((directory) => rm(directory, { force: true, recursive: true }))
+  )
+})
+
+describe("runProjectLinguiCommand", () => {
+  it("compiles from i18n.config.json without a persistent Lingui config", async () => {
+    const directory = await mkdtemp(join(tmpdir(), "workspace-i18n-command-"))
+    temporaryDirectories.push(directory)
+    const projectFilename = join(directory, "i18n.config.json")
+
+    await writeFile(
+      projectFilename,
+      `${JSON.stringify(
+        {
+          catalogPath: "locales/{locale}/messages",
+          include: ["src"],
+          locales: ["en"],
+          sourceLocale: "en",
+        },
+        null,
+        2
+      )}\n`
+    )
+
+    const config = await loadProjectLinguiConfig(projectFilename)
+    const [catalog] = await getCatalogs(config)
+
+    if (!catalog) {
+      throw new Error("Expected a Lingui catalog.")
+    }
+
+    await catalog.write("en", {
+      greeting: {
+        message: "Hello",
+        translation: "Hello",
+      },
+    })
+
+    const result = await runProjectLinguiCommand(projectFilename, "compile")
+    const compiledFilename = join(directory, "locales/en/messages.ts")
+    const firstModifiedTime = (await stat(compiledFilename)).mtimeMs
+
+    expect(result.output).toContain("Compiling message catalogs")
+    expect(await readFile(compiledFilename, "utf8")).toContain(
+      "export const messages"
+    )
+
+    await new Promise((resolve) => setTimeout(resolve, 20))
+    await runProjectLinguiCommand(projectFilename, "compile")
+
+    expect((await stat(compiledFilename)).mtimeMs).toBe(firstModifiedTime)
+  })
+})

packages/i18n/src/cli/lingui-command.ts

@@ -1,6 +1,19 @@
 import { spawn } from "node:child_process"
+import {
+  copyFile,
+  mkdir,
+  mkdtemp,
+  readFile,
+  rm,
+  writeFile,
+} from "node:fs/promises"
+import { tmpdir } from "node:os"
+import { dirname, isAbsolute, join, resolve } from "node:path"
 import { fileURLToPath } from "node:url"
 
+import { createLinguiConfig } from "../config/lingui-config.ts"
+import { getProjectRoot, readProject } from "./project.ts"
+
 const LINGUI_BIN = fileURLToPath(
   new URL("./lingui.js", import.meta.resolve("@lingui/cli"))
 )
@@ -9,12 +22,86 @@ export interface LinguiCommandResult {
   output: string
 }
 
-export async function runLinguiCommand(
+interface RunProjectLinguiCommandOptions {
+  locale?: string
+  typescript?: boolean
+}
+
+interface StagedCompiledCatalog {
+  outputFilename: string
+  stagedOutputFilename: string
+}
+
+function resolveProjectPath(pathname: string, projectRoot: string) {
+  const resolvedRoot = pathname.replaceAll("<rootDir>", projectRoot)
+
+  return isAbsolute(resolvedRoot)
+    ? resolvedRoot
+    : resolve(projectRoot, resolvedRoot)
+}
+
+function resolveLocalePath(pathname: string, locale: string) {
+  return pathname.replaceAll("{locale}", locale)
+}
+
+async function copyFileIfPresent(source: string, destination: string) {
+  await mkdir(dirname(destination), { recursive: true })
+
+  try {
+    await copyFile(source, destination)
+  } catch (error) {
+    if (
+      error &&
+      typeof error === "object" &&
+      "code" in error &&
+      error.code === "ENOENT"
+    ) {
+      return
+    }
+
+    throw error
+  }
+}
+
+async function readFileIfPresent(filename: string) {
+  try {
+    return await readFile(filename, "utf8")
+  } catch (error) {
+    if (
+      error &&
+      typeof error === "object" &&
+      "code" in error &&
+      error.code === "ENOENT"
+    ) {
+      return undefined
+    }
+
+    throw error
+  }
+}
+
+async function publishCompiledCatalogs(
+  catalogs: readonly StagedCompiledCatalog[]
+) {
+  for (const catalog of catalogs) {
+    const nextContent = await readFile(catalog.stagedOutputFilename, "utf8")
+    const currentContent = await readFileIfPresent(catalog.outputFilename)
+
+    if (currentContent === nextContent) {
+      continue
+    }
+
+    await mkdir(dirname(catalog.outputFilename), { recursive: true })
+    await writeFile(catalog.outputFilename, nextContent, "utf8")
+  }
+}
+
+async function runLinguiCommand(
   args: readonly string[],
   cwd: string
 ): Promise<LinguiCommandResult> {
   return new Promise((resolve, reject) => {
-    const child = spawn(process.execPath, [LINGUI_BIN, ...args], {
+    const child = spawn("node", [LINGUI_BIN, ...args], {
       cwd,
       env: process.env,
       stdio: ["ignore", "pipe", "pipe"],
@@ -44,3 +131,89 @@ export async function runLinguiCommand(
     })
   })
 }
+
+export async function runProjectLinguiCommand(
+  projectFilename: string,
+  command: "compile" | "extract",
+  {
+    locale,
+    typescript = command === "compile",
+  }: RunProjectLinguiCommandOptions = {}
+) {
+  const projectRoot = getProjectRoot(projectFilename)
+  const project = await readProject(projectFilename)
+  const temporaryDirectory = await mkdtemp(
+    join(tmpdir(), "workspace-i18n-config-")
+  )
+  const configFilename = join(temporaryDirectory, "lingui.config.mjs")
+  const projectConfig = createLinguiConfig(project)
+  const locales = locale ? [locale] : project.locales
+  const compiledExtension = typescript ? "ts" : "js"
+  const stagedCatalogPath = join(
+    temporaryDirectory,
+    "catalogs",
+    "{locale}",
+    "messages"
+  )
+  const sourceCatalogPath = resolveProjectPath(project.catalogPath, projectRoot)
+  const stagedCompiledCatalogs: StagedCompiledCatalog[] =
+    command === "compile"
+      ? locales.map((catalogLocale) => ({
+          outputFilename: `${resolveLocalePath(sourceCatalogPath, catalogLocale)}.${compiledExtension}`,
+          stagedOutputFilename: `${resolveLocalePath(stagedCatalogPath, catalogLocale)}.${compiledExtension}`,
+        }))
+      : []
+  const config = {
+    ...projectConfig,
+    ...(command === "compile"
+      ? {
+          catalogs: projectConfig.catalogs?.map((catalog) =>
+            typeof catalog === "string"
+              ? catalog
+              : { ...catalog, path: stagedCatalogPath }
+          ),
+        }
+      : undefined),
+    rootDir: projectRoot,
+  }
+
+  try {
+    if (command === "compile") {
+      await Promise.all(
+        locales.map((catalogLocale) =>
+          copyFileIfPresent(
+            `${resolveLocalePath(sourceCatalogPath, catalogLocale)}.po`,
+            `${resolveLocalePath(stagedCatalogPath, catalogLocale)}.po`
+          )
+        )
+      )
+    }
+
+    await writeFile(
+      configFilename,
+      `export default ${JSON.stringify(config, null, 2)}\n`,
+      "utf8"
+    )
+
+    const result = await runLinguiCommand(
+      [
+        command,
+        "--workers",
+        "1",
+        ...(typescript ? ["--typescript"] : []),
+        ...(locale ? ["--locale", locale] : []),
+        "--config",
+        configFilename,
+      ],
+      projectRoot
+    )
+
+    if (command === "compile") {
+      await publishCompiledCatalogs(stagedCompiledCatalogs)
+    }
+
+    return result
+  } finally {
+    await rm(temporaryDirectory, { force: true, recursive: true })
+  }
+}

packages/i18n/src/cli/new-locale.ts

@@ -1,11 +1,5 @@
-import {
-  getLinguiConfigFilename,
-  getProjectRoot,
-  readProject,
-  resolveProjectFilename,
-  writeProject,
-} from "./project"
-import { runLinguiCommand } from "./lingui-command"
+import { readProject, resolveProjectFilename, writeProject } from "./project"
+import { runProjectLinguiCommand } from "./lingui-command"
 
 export interface AddLocaleOptions {
   project?: string
@@ -38,16 +32,9 @@ export async function addLocale(
   }
 
   try {
-    const result = await runLinguiCommand(
-      [
-        "extract",
-        "--locale",
-        locale,
-        "--config",
-        getLinguiConfigFilename(projectFilename),
-      ],
-      getProjectRoot(projectFilename)
-    )
+    const result = await runProjectLinguiCommand(projectFilename, "extract", {
+      locale,
+    })
 
     return {
       created,

packages/i18n/src/cli/project.test.ts

@@ -4,7 +4,7 @@ import { join } from "node:path"
 
 import { afterEach, describe, expect, it } from "vitest"
 
-import { resolveProjectFilename } from "./project"
+import { readProject, resolveProjectFilename } from "./project"
 
 const temporaryDirectories: string[] = []
 
@@ -44,3 +44,34 @@ describe("resolveProjectFilename", () => {
     ).rejects.toThrow("Run the command inside a configured application")
   })
 })
+
+describe("readProject", () => {
+  it("accepts reusable catalog source templates", async () => {
+    const applicationRoot = await mkdtemp(
+      join(tmpdir(), "workspace-i18n-project-")
+    )
+    temporaryDirectories.push(applicationRoot)
+    const projectFilename = join(applicationRoot, "i18n.config.json")
+
+    await writeFile(
+      projectFilename,
+      `${JSON.stringify({
+        catalogPath: "src/locales/{locale}/messages",
+        catalogSources: [
+          "@workspace/ui/locales/{locale}",
+          "@workspace/blocks/navigation/locales/{locale}",
+        ],
+        include: ["src"],
+        locales: ["en", "zh-Hans"],
+        sourceLocale: "en",
+      })}\n`
+    )
+
+    await expect(readProject(projectFilename)).resolves.toMatchObject({
+      catalogSources: [
+        "@workspace/ui/locales/{locale}",
+        "@workspace/blocks/navigation/locales/{locale}",
+      ],
+    })
+  })
+})

packages/i18n/src/cli/project.ts

@@ -2,7 +2,10 @@ import { access, readFile, writeFile } from "node:fs/promises"
 import { dirname, isAbsolute, join, resolve } from "node:path"
 import { fileURLToPath } from "node:url"
 
-import type { I18nProjectConfig } from "../config/lingui-config"
+import {
+  createNormalizedLinguiConfig,
+  type I18nProjectConfig,
+} from "../config/lingui-config.ts"
 
 const PACKAGE_ROOT = fileURLToPath(new URL("../../", import.meta.url))
 const PROJECT_FILENAME = "i18n.config.json"
@@ -31,6 +34,11 @@ function validateProjectConfig(
     !Array.isArray(value.locales) ||
     !value.locales.every((locale) => typeof locale === "string") ||
     typeof value.catalogPath !== "string" ||
+    (value.catalogSources !== undefined &&
+      (!Array.isArray(value.catalogSources) ||
+        !value.catalogSources.every(
+          (catalogSource) => typeof catalogSource === "string"
+        ))) ||
     !Array.isArray(value.include) ||
     !value.include.every((pattern) => typeof pattern === "string") ||
     (value.exclude !== undefined &&
@@ -44,6 +52,7 @@ function validateProjectConfig(
 
   return {
     catalogPath: value.catalogPath,
+    catalogSources: value.catalogSources as string[] | undefined,
     exclude: value.exclude as string[] | undefined,
     include: value.include,
     locales: value.locales,
@@ -115,14 +124,17 @@ export async function writeProject(
   )
 }
 
-export function getLinguiConfigFilename(projectFilename: string) {
-  return join(dirname(projectFilename), "lingui.config.ts")
-}
-
 export function getProjectRoot(projectFilename: string) {
   return dirname(projectFilename)
 }
 
+export async function loadProjectLinguiConfig(projectFilename: string) {
+  return createNormalizedLinguiConfig(
+    await readProject(projectFilename),
+    getProjectRoot(projectFilename)
+  )
+}
+
 export function getPackageRoot() {
   return PACKAGE_ROOT
 }

packages/i18n/src/config/lingui-config.test.ts

@@ -1,12 +1,16 @@
 import { describe, expect, it } from "vitest"
 
-import { createLinguiConfig } from "./lingui-config"
+import {
+  createLinguiConfig,
+  createNormalizedLinguiConfig,
+} from "./lingui-config"
 
 describe("createLinguiConfig", () => {
   it("maps the editable project manifest to a Lingui configuration", () => {
     expect(
       createLinguiConfig({
         catalogPath: "locales/{locale}/messages",
+        catalogSources: ["@workspace/ui/locales/{locale}"],
         exclude: ["**/*.test.ts"],
         include: ["src"],
         locales: ["en", "zh-Hans"],
@@ -27,4 +31,26 @@ describe("createLinguiConfig", () => {
       sourceLocale: "en",
     })
   })
+
+  it("normalizes configuration relative to the consuming application", () => {
+    const config = createNormalizedLinguiConfig(
+      {
+        catalogPath: "locales/{locale}/messages",
+        include: ["src"],
+        locales: ["en"],
+        sourceLocale: "en",
+      },
+      "/workspace/apps/web"
+    )
+
+    expect(config.rootDir).toBe("/workspace/apps/web")
+    expect(config.catalogs[0]).toMatchObject({
+      include: ["/workspace/apps/web/src"],
+      path: "/workspace/apps/web/locales/{locale}/messages",
+    })
+    expect(config.runtimeConfigModule.Trans).toEqual([
+      "@workspace/i18n",
+      "Translate",
+    ])
+  })
 })

packages/i18n/src/config/lingui-config.ts

@@ -1,9 +1,14 @@
 import { isAbsolute } from "node:path"
 
-import type { LinguiConfig } from "@lingui/conf"
+import {
+  makeConfig,
+  type LinguiConfig,
+  type LinguiConfigNormalized,
+} from "@lingui/conf"
 
 export interface I18nProjectConfig {
   catalogPath: string
+  catalogSources?: readonly string[]
   exclude?: readonly string[]
   include: readonly string[]
   locales: readonly string[]
@@ -34,3 +39,13 @@ export function createLinguiConfig(project: I18nProjectConfig): LinguiConfig {
     sourceLocale: project.sourceLocale,
   }
 }
+
+export function createNormalizedLinguiConfig(
+  project: I18nProjectConfig,
+  rootDir: string
+): LinguiConfigNormalized {
+  return makeConfig({
+    ...createLinguiConfig(project),
+    rootDir,
+  })
+}

packages/i18n/src/devtool/ansi-text.test.tsx

@@ -0,0 +1,29 @@
+import * as React from "react"
+import { renderToStaticMarkup } from "react-dom/server"
+import { describe, expect, it } from "vitest"
+
+import { AnsiText, resolveAnsiComponent } from "./ansi-text"
+
+describe("AnsiText", () => {
+  it("resolves direct, CommonJS, and nested default exports", () => {
+    const Component = ({ children }: { children?: string }) => (
+      <code>{children}</code>
+    )
+
+    expect(resolveAnsiComponent(Component)).toBe(Component)
+    expect(resolveAnsiComponent({ default: Component })).toBe(Component)
+    expect(resolveAnsiComponent({ default: { default: Component } })).toBe(
+      Component
+    )
+  })
+
+  it("renders ANSI SGR output during server rendering", () => {
+    const markup = renderToStaticMarkup(
+      <AnsiText>{"\u001b[32mDone\u001b[39m"}</AnsiText>
+    )
+
+    expect(markup).toContain("Done")
+    expect(markup).toContain("color:")
+    expect(markup).not.toContain("\u001b")
+  })
+})

packages/i18n/src/devtool/ansi-text.tsx

@@ -0,0 +1,39 @@
+import * as React from "react"
+import * as AnsiModule from "ansi-to-react"
+
+export interface AnsiTextProps {
+  children?: string
+  className?: string
+  linkify?: boolean | "fuzzy"
+  useClasses?: boolean
+}
+
+type AnsiComponent = React.ComponentType<AnsiTextProps>
+
+export function resolveAnsiComponent(moduleValue: unknown): AnsiComponent {
+  let candidate = moduleValue
+
+  for (let depth = 0; depth < 3; depth += 1) {
+    if (typeof candidate === "function") {
+      return candidate as AnsiComponent
+    }
+
+    if (
+      typeof candidate !== "object" ||
+      candidate === null ||
+      !("default" in candidate)
+    ) {
+      break
+    }
+
+    candidate = candidate.default
+  }
+
+  throw new TypeError("ansi-to-react did not export a React component.")
+}
+
+const AnsiComponent = resolveAnsiComponent(AnsiModule)
+
+export function AnsiText(props: AnsiTextProps) {
+  return <AnsiComponent {...props} />
+}

packages/i18n/src/devtool/api-client.ts

@@ -1,5 +1,10 @@
 export type DevtoolAction = "compile" | "extract"
 
+export interface DevtoolProject {
+  locales: readonly string[]
+  sourceLocale: string
+}
+
 interface ApiError {
   error?: string
 }
@@ -29,3 +34,9 @@ export async function runDevtoolAction(
     { method: "POST" }
   )
 }
+
+export async function requestDevtoolProject(baseUrl = "/__i18n") {
+  const normalizedBaseUrl = baseUrl.replace(/\/+$/, "")
+
+  return requestDevtoolJson<DevtoolProject>(`${normalizedBaseUrl}/project`)
+}

packages/i18n/src/devtool/app/app.tsx

@@ -40,19 +40,26 @@ export function DevtoolApp() {
     )
   }
 
-  const locale = state.project.locales[0]
+  const locale =
+    state.project.locales.find(
+      (locale) => locale !== state.project.sourceLocale
+    ) ?? state.project.sourceLocale
 
-  if (!locale) {
+  if (!state.project.locales.includes(locale)) {
     return (
       <main className="devtool-app__state">
-        Add a locale with <code>workspace-i18n new &lt;locale&gt;</code>.
+        Add a target locale with <code>workspace-i18n new &lt;locale&gt;</code>.
       </main>
     )
   }
 
   return (
     <I18nProvider locale={locale} locales={state.project.locales}>
-      <I18nDevtool defaultOpen mode="standalone" />
+      <I18nDevtool
+        defaultOpen
+        mode="standalone"
+        sourceLocale={state.project.sourceLocale}
+      />
     </I18nProvider>
   )
 }

packages/i18n/src/devtool/devtool-action-output.tsx

@@ -0,0 +1,65 @@
+import {
+  ChevronDownIcon,
+  ChevronUpIcon,
+  TerminalIcon,
+  XIcon,
+} from "lucide-react"
+
+import { AnsiText } from "./ansi-text"
+import { useDevtoolLocalization } from "./devtool-localization"
+
+export interface DevtoolActionOutputProps {
+  error?: boolean
+  expanded: boolean
+  onClose: () => void
+  onExpandedChange: (expanded: boolean) => void
+  output: string
+}
+
+export function DevtoolActionOutput({
+  error = false,
+  expanded,
+  onClose,
+  onExpandedChange,
+  output,
+}: DevtoolActionOutputProps) {
+  const ToggleIcon = expanded ? ChevronUpIcon : ChevronDownIcon
+  const { messages } = useDevtoolLocalization()
+
+  return (
+    <section className="i18n-devtool__output" data-error={error || undefined}>
+      <header className="i18n-devtool__output-header">
+        <span>
+          <TerminalIcon aria-hidden="true" />
+          {messages.commandOutput.title}
+        </span>
+        <div>
+          <button
+            type="button"
+            aria-expanded={expanded}
+            aria-label={
+              expanded
+                ? messages.commandOutput.collapse
+                : messages.commandOutput.expand
+            }
+            onClick={() => onExpandedChange(!expanded)}
+          >
+            <ToggleIcon aria-hidden="true" />
+          </button>
+          <button
+            type="button"
+            aria-label={messages.commandOutput.close}
+            onClick={onClose}
+          >
+            <XIcon aria-hidden="true" />
+          </button>
+        </div>
+      </header>
+      {expanded && (
+        <pre>
+          <AnsiText>{output}</AnsiText>
+        </pre>
+      )}
+    </section>
+  )
+}

packages/i18n/src/devtool/devtool-locale-select.tsx

@@ -0,0 +1,50 @@
+import type { LocaleDefinition } from "../runtime"
+import { useDevtoolLocalization } from "./devtool-localization"
+import { DevtoolSelect } from "./devtool-select"
+
+export interface DevtoolLocaleSelectProps {
+  locale: string
+  locales: readonly LocaleDefinition[]
+  onLocaleChange: (locale: string) => void
+  sourceLocaleLabel?: string
+}
+
+export function DevtoolLocaleSelect({
+  locale,
+  locales,
+  onLocaleChange,
+  sourceLocaleLabel,
+}: DevtoolLocaleSelectProps) {
+  const { messages } = useDevtoolLocalization()
+
+  return (
+    <>
+      {sourceLocaleLabel && (
+        <div className="i18n-devtool__source-locale-field">
+          <span>{messages.locale.sourceLabel}</span>
+          <output className="i18n-devtool__source-locale">
+            {sourceLocaleLabel}
+          </output>
+        </div>
+      )}
+      <label className="i18n-devtool__locale-field">
+        <span>{messages.locale.label}</span>
+        <DevtoolSelect
+          aria-label={messages.locale.controlLabel}
+          className="i18n-devtool__locale-select"
+          disabled={locales.length === 0}
+          options={
+            locales.length === 0
+              ? [{ label: messages.locale.noTargetLocales, value: "" }]
+              : locales.map((definition) => ({
+                  label: definition.label ?? definition.locale,
+                  value: definition.locale,
+                }))
+          }
+          value={locale}
+          onChange={(event) => onLocaleChange(event.target.value)}
+        />
+      </label>
+    </>
+  )
+}

packages/i18n/src/devtool/devtool-localization.test.ts

@@ -0,0 +1,21 @@
+import { describe, expect, it } from "vitest"
+
+import { resolveI18nDevtoolLocale } from "./devtool-localization"
+
+describe("resolveI18nDevtoolLocale", () => {
+  it("normalizes common BCP 47 language tags", () => {
+    expect(resolveI18nDevtoolLocale("zh-CN")).toBe("zh-Hans")
+    expect(resolveI18nDevtoolLocale("zh-TW")).toBe("zh-Hant")
+    expect(resolveI18nDevtoolLocale("es-419")).toBe("es")
+  })
+
+  it("selects the first supported preferred locale", () => {
+    expect(
+      resolveI18nDevtoolLocale(undefined, ["pt-BR", "fr-CA", "en-US"])
+    ).toBe("fr")
+  })
+
+  it("falls back to English", () => {
+    expect(resolveI18nDevtoolLocale(undefined, ["pt-BR"])).toBe("en")
+  })
+})

packages/i18n/src/devtool/devtool-localization.tsx

@@ -0,0 +1,933 @@
+import * as React from "react"
+
+export const I18N_DEVTOOL_LOCALES = [
+  "en",
+  "de",
+  "es",
+  "fr",
+  "ja",
+  "ko",
+  "zh-Hans",
+  "zh-Hant",
+] as const
+
+export type I18nDevtoolLocale = (typeof I18N_DEVTOOL_LOCALES)[number]
+
+interface DevtoolMessages {
+  actions: {
+    compile: string
+    compileCompleted: string
+    compileFailed: string
+    compiling: string
+    enterFullscreen: string
+    exitFullscreen: string
+    extract: string
+    extractCompleted: string
+    extractFailed: string
+    extracting: string
+    label: string
+  }
+  commandOutput: {
+    close: string
+    collapse: string
+    expand: string
+    title: string
+  }
+  locale: {
+    controlLabel: string
+    label: string
+    noTargetLocales: string
+    sourceLabel: string
+  }
+  messagePanel: {
+    additionalSourceLocations: (count: number) => string
+    chooseCopyContent: string
+    collapseNamespace: (path: string) => string
+    collapsePackage: (packageName: string) => string
+    copied: string
+    copyDescriptor: string
+    copyFailed: string
+    copyMessageId: string
+    copySource: string
+    copySourceLocation: string
+    copyTranslation: string
+    couldNotSave: string
+    emptyCatalog: string
+    emptyPackage: string
+    expandNamespace: (path: string) => string
+    expandPackage: (packageName: string) => string
+    loading: (locale: string) => string
+    messageCount: (count: number) => string
+    messageEditor: string
+    messageNavigation: string
+    missingCount: (count: number) => string
+    missingOnly: string
+    noMatches: string
+    noMissingMessages: string
+    noSearchResults: string
+    reset: string
+    retry: string
+    save: string
+    saved: string
+    saving: string
+    search: string
+    selectMessage: string
+    source: string
+    sourceLocations: string
+    translation: string
+    translationFor: (id: string, locale: string) => string
+    unknownSourceLocation: string
+  }
+  settings: {
+    auto: string
+    autoTheme: string
+    center: string
+    dark: string
+    darkTheme: string
+    interfaceLanguage: string
+    interfaceLanguageControl: string
+    label: string
+    left: string
+    light: string
+    lightTheme: string
+    open: string
+    panelPosition: string
+    panelPositionControl: string
+    right: string
+    theme: string
+    themeControl: string
+  }
+  trigger: {
+    close: string
+    open: string
+  }
+}
+
+const messagesByLocale: Record<I18nDevtoolLocale, DevtoolMessages> = {
+  en: {
+    actions: {
+      compile: "Compile",
+      compileCompleted: "Compilation completed.",
+      compileFailed: "Compilation failed.",
+      compiling: "Compiling…",
+      enterFullscreen: "Enter full screen",
+      exitFullscreen: "Exit full screen",
+      extract: "Extract",
+      extractCompleted: "Extraction completed.",
+      extractFailed: "Extraction failed.",
+      extracting: "Extracting…",
+      label: "Actions",
+    },
+    commandOutput: {
+      close: "Close command output",
+      collapse: "Collapse command output",
+      expand: "Expand command output",
+      title: "Command output",
+    },
+    locale: {
+      controlLabel: "Target message locale",
+      label: "Target locale",
+      noTargetLocales: "No target locales configured.",
+      sourceLabel: "Source locale",
+    },
+    messagePanel: {
+      additionalSourceLocations: (count) =>
+        `${count} additional source locations`,
+      chooseCopyContent: "Choose content to copy",
+      collapseNamespace: (path) => `Collapse ${path}`,
+      collapsePackage: (packageName) => `Collapse ${packageName}`,
+      copied: "Copied",
+      copyDescriptor: "Copy message descriptor",
+      copyFailed: "Could not copy",
+      copyMessageId: "Copy message ID",
+      copySource: "Copy source text",
+      copySourceLocation: "Copy source location",
+      copyTranslation: "Copy current translation",
+      couldNotSave: "Could not save",
+      emptyCatalog:
+        "No messages are available yet. Extract the project to populate this catalog.",
+      emptyPackage: "No messages were found in this package.",
+      expandNamespace: (path) => `Expand ${path}`,
+      expandPackage: (packageName) => `Expand ${packageName}`,
+      loading: (locale) => `Loading ${locale}…`,
+      messageCount: (count) => `${count} messages`,
+      messageEditor: "Message editor",
+      messageNavigation: "Message navigation",
+      missingCount: (count) => `${count} missing`,
+      missingOnly: "Missing only",
+      noMatches: "No messages match the current filters.",
+      noMissingMessages: "All messages have translations.",
+      noSearchResults: "No messages match your search.",
+      reset: "Reset",
+      retry: "Retry",
+      save: "Save",
+      saved: "Saved",
+      saving: "Saving…",
+      search: "Search messages",
+      selectMessage: "Select a message to view and edit its translation.",
+      source: "Source",
+      sourceLocations: "Source locations",
+      translation: "Translation",
+      translationFor: (id, locale) => `${id} translation for ${locale}`,
+      unknownSourceLocation: "Unknown source location",
+    },
+    settings: {
+      auto: "Auto",
+      autoTheme: "Auto theme",
+      center: "Center",
+      dark: "Dark",
+      darkTheme: "Dark theme",
+      interfaceLanguage: "Interface language",
+      interfaceLanguageControl: "I18n Devtool interface language",
+      label: "I18n Devtool settings",
+      left: "Left",
+      light: "Light",
+      lightTheme: "Light theme",
+      open: "Open I18n Devtool settings",
+      panelPosition: "Panel position",
+      panelPositionControl: "I18n Devtool panel position",
+      right: "Right",
+      theme: "Theme",
+      themeControl: "I18n Devtool theme",
+    },
+    trigger: {
+      close: "Close I18n Devtool",
+      open: "Open I18n Devtool",
+    },
+  },
+  de: {
+    actions: {
+      compile: "Kompilieren",
+      compileCompleted: "Kompilierung abgeschlossen.",
+      compileFailed: "Kompilierung fehlgeschlagen.",
+      compiling: "Wird kompiliert…",
+      enterFullscreen: "Vollbildmodus aktivieren",
+      exitFullscreen: "Vollbildmodus beenden",
+      extract: "Extrahieren",
+      extractCompleted: "Extraktion abgeschlossen.",
+      extractFailed: "Extraktion fehlgeschlagen.",
+      extracting: "Wird extrahiert…",
+      label: "Aktionen",
+    },
+    commandOutput: {
+      close: "Befehlsausgabe schließen",
+      collapse: "Befehlsausgabe einklappen",
+      expand: "Befehlsausgabe ausklappen",
+      title: "Befehlsausgabe",
+    },
+    locale: {
+      controlLabel: "Zielsprache der Nachrichten",
+      label: "Zielsprache",
+      noTargetLocales: "Keine Zielsprachen konfiguriert.",
+      sourceLabel: "Ausgangssprache",
+    },
+    messagePanel: {
+      additionalSourceLocations: (count) => `${count} weitere Quellorte`,
+      chooseCopyContent: "Inhalt zum Kopieren auswählen",
+      collapseNamespace: (path) => `${path} einklappen`,
+      collapsePackage: (packageName) => `${packageName} einklappen`,
+      copied: "Kopiert",
+      copyDescriptor: "Nachrichten-Deskriptor kopieren",
+      copyFailed: "Kopieren fehlgeschlagen",
+      copyMessageId: "Nachrichten-ID kopieren",
+      copySource: "Quelltext kopieren",
+      copySourceLocation: "Quellort kopieren",
+      copyTranslation: "Aktuelle Übersetzung kopieren",
+      couldNotSave: "Speichern fehlgeschlagen",
+      emptyCatalog:
+        "Es sind noch keine Nachrichten verfügbar. Extrahieren Sie das Projekt, um diesen Katalog zu füllen.",
+      emptyPackage: "In diesem Paket wurden keine Nachrichten gefunden.",
+      expandNamespace: (path) => `${path} ausklappen`,
+      expandPackage: (packageName) => `${packageName} ausklappen`,
+      loading: (locale) => `${locale} wird geladen…`,
+      messageCount: (count) => `${count} Nachrichten`,
+      messageEditor: "Nachrichteneditor",
+      messageNavigation: "Nachrichtennavigation",
+      missingCount: (count) => `${count} fehlen`,
+      missingOnly: "Nur fehlende",
+      noMatches: "Keine Nachrichten entsprechen den aktuellen Filtern.",
+      noMissingMessages: "Alle Nachrichten sind übersetzt.",
+      noSearchResults: "Keine Nachrichten entsprechen Ihrer Suche.",
+      reset: "Zurücksetzen",
+      retry: "Erneut versuchen",
+      save: "Speichern",
+      saved: "Gespeichert",
+      saving: "Wird gespeichert…",
+      search: "Nachrichten durchsuchen",
+      selectMessage:
+        "Wählen Sie eine Nachricht aus, um ihre Übersetzung anzuzeigen und zu bearbeiten.",
+      source: "Quelle",
+      sourceLocations: "Quellorte",
+      translation: "Übersetzung",
+      translationFor: (id, locale) => `${id}: Übersetzung für ${locale}`,
+      unknownSourceLocation: "Unbekannter Quellort",
+    },
+    settings: {
+      auto: "Automatisch",
+      autoTheme: "Automatisches Design",
+      center: "Mitte",
+      dark: "Dunkel",
+      darkTheme: "Dunkles Design",
+      interfaceLanguage: "Oberflächensprache",
+      interfaceLanguageControl: "Oberflächensprache des I18n Devtools",
+      label: "I18n-Devtool-Einstellungen",
+      left: "Links",
+      light: "Hell",
+      lightTheme: "Helles Design",
+      open: "I18n-Devtool-Einstellungen öffnen",
+      panelPosition: "Panelposition",
+      panelPositionControl: "Position des I18n-Devtool-Panels",
+      right: "Rechts",
+      theme: "Design",
+      themeControl: "I18n-Devtool-Design",
+    },
+    trigger: {
+      close: "I18n Devtool schließen",
+      open: "I18n Devtool öffnen",
+    },
+  },
+  es: {
+    actions: {
+      compile: "Compilar",
+      compileCompleted: "Compilación completada.",
+      compileFailed: "Error de compilación.",
+      compiling: "Compilando…",
+      enterFullscreen: "Entrar en pantalla completa",
+      exitFullscreen: "Salir de pantalla completa",
+      extract: "Extraer",
+      extractCompleted: "Extracción completada.",
+      extractFailed: "Error de extracción.",
+      extracting: "Extrayendo…",
+      label: "Acciones",
+    },
+    commandOutput: {
+      close: "Cerrar salida del comando",
+      collapse: "Contraer salida del comando",
+      expand: "Expandir salida del comando",
+      title: "Salida del comando",
+    },
+    locale: {
+      controlLabel: "Idioma de destino de los mensajes",
+      label: "Idioma de destino",
+      noTargetLocales: "No hay idiomas de destino configurados.",
+      sourceLabel: "Idioma de origen",
+    },
+    messagePanel: {
+      additionalSourceLocations: (count) =>
+        `${count} ubicaciones de origen adicionales`,
+      chooseCopyContent: "Elegir contenido para copiar",
+      collapseNamespace: (path) => `Contraer ${path}`,
+      collapsePackage: (packageName) => `Contraer ${packageName}`,
+      copied: "Copiado",
+      copyDescriptor: "Copiar descriptor del mensaje",
+      copyFailed: "No se pudo copiar",
+      copyMessageId: "Copiar ID del mensaje",
+      copySource: "Copiar texto de origen",
+      copySourceLocation: "Copiar ubicación de origen",
+      copyTranslation: "Copiar traducción actual",
+      couldNotSave: "No se pudo guardar",
+      emptyCatalog:
+        "Aún no hay mensajes disponibles. Extraiga el proyecto para completar este catálogo.",
+      emptyPackage: "No se encontraron mensajes en este paquete.",
+      expandNamespace: (path) => `Expandir ${path}`,
+      expandPackage: (packageName) => `Expandir ${packageName}`,
+      loading: (locale) => `Cargando ${locale}…`,
+      messageCount: (count) => `${count} mensajes`,
+      messageEditor: "Editor de mensajes",
+      messageNavigation: "Navegación de mensajes",
+      missingCount: (count) => `${count} faltantes`,
+      missingOnly: "Solo faltantes",
+      noMatches: "Ningún mensaje coincide con los filtros actuales.",
+      noMissingMessages: "Todos los mensajes tienen traducción.",
+      noSearchResults: "Ningún mensaje coincide con la búsqueda.",
+      reset: "Restablecer",
+      retry: "Reintentar",
+      save: "Guardar",
+      saved: "Guardado",
+      saving: "Guardando…",
+      search: "Buscar mensajes",
+      selectMessage: "Seleccione un mensaje para ver y editar su traducción.",
+      source: "Origen",
+      sourceLocations: "Ubicaciones de origen",
+      translation: "Traducción",
+      translationFor: (id, locale) => `Traducción de ${id} para ${locale}`,
+      unknownSourceLocation: "Ubicación de origen desconocida",
+    },
+    settings: {
+      auto: "Automático",
+      autoTheme: "Tema automático",
+      center: "Centro",
+      dark: "Oscuro",
+      darkTheme: "Tema oscuro",
+      interfaceLanguage: "Idioma de la interfaz",
+      interfaceLanguageControl: "Idioma de la interfaz de I18n Devtool",
+      label: "Ajustes de I18n Devtool",
+      left: "Izquierda",
+      light: "Claro",
+      lightTheme: "Tema claro",
+      open: "Abrir ajustes de I18n Devtool",
+      panelPosition: "Posición del panel",
+      panelPositionControl: "Posición del panel de I18n Devtool",
+      right: "Derecha",
+      theme: "Tema",
+      themeControl: "Tema de I18n Devtool",
+    },
+    trigger: {
+      close: "Cerrar I18n Devtool",
+      open: "Abrir I18n Devtool",
+    },
+  },
+  fr: {
+    actions: {
+      compile: "Compiler",
+      compileCompleted: "Compilation terminée.",
+      compileFailed: "Échec de la compilation.",
+      compiling: "Compilation…",
+      enterFullscreen: "Passer en plein écran",
+      exitFullscreen: "Quitter le plein écran",
+      extract: "Extraire",
+      extractCompleted: "Extraction terminée.",
+      extractFailed: "Échec de l’extraction.",
+      extracting: "Extraction…",
+      label: "Actions",
+    },
+    commandOutput: {
+      close: "Fermer la sortie de commande",
+      collapse: "Réduire la sortie de commande",
+      expand: "Développer la sortie de commande",
+      title: "Sortie de commande",
+    },
+    locale: {
+      controlLabel: "Langue cible des messages",
+      label: "Langue cible",
+      noTargetLocales: "Aucune langue cible n’est configurée.",
+      sourceLabel: "Langue source",
+    },
+    messagePanel: {
+      additionalSourceLocations: (count) =>
+        `${count} emplacements source supplémentaires`,
+      chooseCopyContent: "Choisir le contenu à copier",
+      collapseNamespace: (path) => `Réduire ${path}`,
+      collapsePackage: (packageName) => `Réduire ${packageName}`,
+      copied: "Copié",
+      copyDescriptor: "Copier le descripteur du message",
+      copyFailed: "Échec de la copie",
+      copyMessageId: "Copier l’identifiant du message",
+      copySource: "Copier le texte source",
+      copySourceLocation: "Copier l’emplacement source",
+      copyTranslation: "Copier la traduction actuelle",
+      couldNotSave: "Échec de l’enregistrement",
+      emptyCatalog:
+        "Aucun message n’est encore disponible. Extrayez le projet pour remplir ce catalogue.",
+      emptyPackage: "Aucun message n’a été trouvé dans ce package.",
+      expandNamespace: (path) => `Développer ${path}`,
+      expandPackage: (packageName) => `Développer ${packageName}`,
+      loading: (locale) => `Chargement de ${locale}…`,
+      messageCount: (count) => `${count} messages`,
+      messageEditor: "Éditeur de messages",
+      messageNavigation: "Navigation des messages",
+      missingCount: (count) => `${count} manquants`,
+      missingOnly: "Manquants uniquement",
+      noMatches: "Aucun message ne correspond aux filtres actuels.",
+      noMissingMessages: "Tous les messages sont traduits.",
+      noSearchResults: "Aucun message ne correspond à votre recherche.",
+      reset: "Réinitialiser",
+      retry: "Réessayer",
+      save: "Enregistrer",
+      saved: "Enregistré",
+      saving: "Enregistrement…",
+      search: "Rechercher des messages",
+      selectMessage:
+        "Sélectionnez un message pour afficher et modifier sa traduction.",
+      source: "Source",
+      sourceLocations: "Emplacements source",
+      translation: "Traduction",
+      translationFor: (id, locale) => `Traduction de ${id} pour ${locale}`,
+      unknownSourceLocation: "Emplacement source inconnu",
+    },
+    settings: {
+      auto: "Auto",
+      autoTheme: "Thème automatique",
+      center: "Centre",
+      dark: "Sombre",
+      darkTheme: "Thème sombre",
+      interfaceLanguage: "Langue de l’interface",
+      interfaceLanguageControl: "Langue de l’interface de l’outil I18n",
+      label: "Paramètres de I18n Devtool",
+      left: "Gauche",
+      light: "Clair",
+      lightTheme: "Thème clair",
+      open: "Ouvrir les paramètres de I18n Devtool",
+      panelPosition: "Position du panneau",
+      panelPositionControl: "Position du panneau I18n Devtool",
+      right: "Droite",
+      theme: "Thème",
+      themeControl: "Thème de I18n Devtool",
+    },
+    trigger: {
+      close: "Fermer I18n Devtool",
+      open: "Ouvrir I18n Devtool",
+    },
+  },
+  ja: {
+    actions: {
+      compile: "コンパイル",
+      compileCompleted: "コンパイルが完了しました。",
+      compileFailed: "コンパイルに失敗しました。",
+      compiling: "コンパイル中…",
+      enterFullscreen: "全画面表示にする",
+      exitFullscreen: "全画面表示を終了",
+      extract: "抽出",
+      extractCompleted: "抽出が完了しました。",
+      extractFailed: "抽出に失敗しました。",
+      extracting: "抽出中…",
+      label: "操作",
+    },
+    commandOutput: {
+      close: "コマンド出力を閉じる",
+      collapse: "コマンド出力を折りたたむ",
+      expand: "コマンド出力を展開する",
+      title: "コマンド出力",
+    },
+    locale: {
+      controlLabel: "メッセージの翻訳先言語",
+      label: "翻訳先言語",
+      noTargetLocales: "翻訳先言語が設定されていません。",
+      sourceLabel: "原文言語",
+    },
+    messagePanel: {
+      additionalSourceLocations: (count) => `他 ${count} 件のソース位置`,
+      chooseCopyContent: "コピーする内容を選択",
+      collapseNamespace: (path) => `${path} を折りたたむ`,
+      collapsePackage: (packageName) => `${packageName} を折りたたむ`,
+      copied: "コピーしました",
+      copyDescriptor: "メッセージ記述子をコピー",
+      copyFailed: "コピーできませんでした",
+      copyMessageId: "メッセージ ID をコピー",
+      copySource: "原文をコピー",
+      copySourceLocation: "ソース位置をコピー",
+      copyTranslation: "現在の翻訳をコピー",
+      couldNotSave: "保存できませんでした",
+      emptyCatalog:
+        "メッセージはまだありません。プロジェクトを抽出してカタログを作成してください。",
+      emptyPackage: "このパッケージにはメッセージがありません。",
+      expandNamespace: (path) => `${path} を展開`,
+      expandPackage: (packageName) => `${packageName} を展開`,
+      loading: (locale) => `${locale} を読み込み中…`,
+      messageCount: (count) => `${count} 件`,
+      messageEditor: "メッセージエディター",
+      messageNavigation: "メッセージナビゲーション",
+      missingCount: (count) => `未翻訳 ${count} 件`,
+      missingOnly: "未翻訳のみ",
+      noMatches: "現在のフィルターに一致するメッセージはありません。",
+      noMissingMessages: "すべてのメッセージが翻訳済みです。",
+      noSearchResults: "検索に一致するメッセージはありません。",
+      reset: "リセット",
+      retry: "再試行",
+      save: "保存",
+      saved: "保存済み",
+      saving: "保存中…",
+      search: "メッセージを検索",
+      selectMessage: "翻訳を表示・編集するメッセージを選択してください。",
+      source: "原文",
+      sourceLocations: "ソース位置",
+      translation: "翻訳",
+      translationFor: (id, locale) => `${id} の ${locale} 翻訳`,
+      unknownSourceLocation: "ソース位置不明",
+    },
+    settings: {
+      auto: "自動",
+      autoTheme: "自動テーマ",
+      center: "中央",
+      dark: "ダーク",
+      darkTheme: "ダークテーマ",
+      interfaceLanguage: "表示言語",
+      interfaceLanguageControl: "I18n Devtool の表示言語",
+      label: "I18n Devtool の設定",
+      left: "左",
+      light: "ライト",
+      lightTheme: "ライトテーマ",
+      open: "I18n Devtool の設定を開く",
+      panelPosition: "パネル位置",
+      panelPositionControl: "I18n Devtool のパネル位置",
+      right: "右",
+      theme: "テーマ",
+      themeControl: "I18n Devtool のテーマ",
+    },
+    trigger: {
+      close: "I18n Devtool を閉じる",
+      open: "I18n Devtool を開く",
+    },
+  },
+  ko: {
+    actions: {
+      compile: "컴파일",
+      compileCompleted: "컴파일이 완료되었습니다.",
+      compileFailed: "컴파일에 실패했습니다.",
+      compiling: "컴파일 중…",
+      enterFullscreen: "전체 화면으로 전환",
+      exitFullscreen: "전체 화면 종료",
+      extract: "추출",
+      extractCompleted: "추출이 완료되었습니다.",
+      extractFailed: "추출에 실패했습니다.",
+      extracting: "추출 중…",
+      label: "작업",
+    },
+    commandOutput: {
+      close: "명령 출력을 닫기",
+      collapse: "명령 출력을 접기",
+      expand: "명령 출력을 펼치기",
+      title: "명령 출력",
+    },
+    locale: {
+      controlLabel: "메시지 대상 언어",
+      label: "대상 언어",
+      noTargetLocales: "대상 언어가 설정되지 않았습니다.",
+      sourceLabel: "원본 언어",
+    },
+    messagePanel: {
+      additionalSourceLocations: (count) => `추가 소스 위치 ${count}개`,
+      chooseCopyContent: "복사할 내용 선택",
+      collapseNamespace: (path) => `${path} 접기`,
+      collapsePackage: (packageName) => `${packageName} 접기`,
+      copied: "복사됨",
+      copyDescriptor: "메시지 설명자 복사",
+      copyFailed: "복사하지 못했습니다",
+      copyMessageId: "메시지 ID 복사",
+      copySource: "원문 복사",
+      copySourceLocation: "소스 위치 복사",
+      copyTranslation: "현재 번역 복사",
+      couldNotSave: "저장하지 못했습니다",
+      emptyCatalog:
+        "아직 사용할 수 있는 메시지가 없습니다. 프로젝트에서 메시지를 추출해 카탈로그를 채우세요.",
+      emptyPackage: "이 패키지에는 메시지가 없습니다.",
+      expandNamespace: (path) => `${path} 펼치기`,
+      expandPackage: (packageName) => `${packageName} 펼치기`,
+      loading: (locale) => `${locale} 불러오는 중…`,
+      messageCount: (count) => `${count}개 메시지`,
+      messageEditor: "메시지 편집기",
+      messageNavigation: "메시지 탐색",
+      missingCount: (count) => `${count}개 누락`,
+      missingOnly: "누락된 항목만",
+      noMatches: "현재 필터와 일치하는 메시지가 없습니다.",
+      noMissingMessages: "모든 메시지가 번역되었습니다.",
+      noSearchResults: "검색어와 일치하는 메시지가 없습니다.",
+      reset: "재설정",
+      retry: "다시 시도",
+      save: "저장",
+      saved: "저장됨",
+      saving: "저장 중…",
+      search: "메시지 검색",
+      selectMessage: "번역을 확인하고 편집할 메시지를 선택하세요.",
+      source: "원문",
+      sourceLocations: "소스 위치",
+      translation: "번역",
+      translationFor: (id, locale) => `${id}의 ${locale} 번역`,
+      unknownSourceLocation: "알 수 없는 소스 위치",
+    },
+    settings: {
+      auto: "자동",
+      autoTheme: "자동 테마",
+      center: "가운데",
+      dark: "다크",
+      darkTheme: "다크 테마",
+      interfaceLanguage: "인터페이스 언어",
+      interfaceLanguageControl: "I18n Devtool 인터페이스 언어",
+      label: "I18n Devtool 설정",
+      left: "왼쪽",
+      light: "라이트",
+      lightTheme: "라이트 테마",
+      open: "I18n Devtool 설정 열기",
+      panelPosition: "패널 위치",
+      panelPositionControl: "I18n Devtool 패널 위치",
+      right: "오른쪽",
+      theme: "테마",
+      themeControl: "I18n Devtool 테마",
+    },
+    trigger: {
+      close: "I18n Devtool 닫기",
+      open: "I18n Devtool 열기",
+    },
+  },
+  "zh-Hans": {
+    actions: {
+      compile: "编译",
+      compileCompleted: "编译完成。",
+      compileFailed: "编译失败。",
+      compiling: "正在编译…",
+      enterFullscreen: "进入全屏",
+      exitFullscreen: "退出全屏",
+      extract: "提取",
+      extractCompleted: "提取完成。",
+      extractFailed: "提取失败。",
+      extracting: "正在提取…",
+      label: "操作",
+    },
+    commandOutput: {
+      close: "关闭命令输出",
+      collapse: "收起命令输出",
+      expand: "展开命令输出",
+      title: "命令输出",
+    },
+    locale: {
+      controlLabel: "消息目标语言",
+      label: "目标语言",
+      noTargetLocales: "尚未配置目标语言。",
+      sourceLabel: "源语言",
+    },
+    messagePanel: {
+      additionalSourceLocations: (count) => `另外 ${count} 个来源位置`,
+      chooseCopyContent: "选择要复制的内容",
+      collapseNamespace: (path) => `折叠 ${path}`,
+      collapsePackage: (packageName) => `折叠 ${packageName}`,
+      copied: "已复制",
+      copyDescriptor: "复制 Message Descriptor",
+      copyFailed: "复制失败",
+      copyMessageId: "复制 Message ID",
+      copySource: "复制原文",
+      copySourceLocation: "复制来源位置",
+      copyTranslation: "复制当前译文",
+      couldNotSave: "保存失败",
+      emptyCatalog: "尚无可用消息。请先提取项目消息以生成目录。",
+      emptyPackage: "这个包中暂无消息。",
+      expandNamespace: (path) => `展开 ${path}`,
+      expandPackage: (packageName) => `展开 ${packageName}`,
+      loading: (locale) => `正在加载 ${locale}…`,
+      messageCount: (count) => `共 ${count} 条`,
+      messageEditor: "消息编辑器",
+      messageNavigation: "消息导航",
+      missingCount: (count) => `缺失 ${count} 条`,
+      missingOnly: "仅显示缺失项",
+      noMatches: "没有符合当前筛选条件的消息。",
+      noMissingMessages: "所有消息均已完成翻译。",
+      noSearchResults: "没有与搜索内容匹配的消息。",
+      reset: "重置",
+      retry: "重试",
+      save: "保存",
+      saved: "已保存",
+      saving: "正在保存…",
+      search: "搜索消息",
+      selectMessage: "请选择一条消息以查看和编辑译文。",
+      source: "原文",
+      sourceLocations: "来源位置",
+      translation: "译文",
+      translationFor: (id, locale) => `${id} 的 ${locale} 翻译`,
+      unknownSourceLocation: "未知来源位置",
+    },
+    settings: {
+      auto: "自动",
+      autoTheme: "自动主题",
+      center: "居中",
+      dark: "深色",
+      darkTheme: "深色主题",
+      interfaceLanguage: "界面语言",
+      interfaceLanguageControl: "I18n 开发工具界面语言",
+      label: "I18n 开发工具设置",
+      left: "左侧",
+      light: "浅色",
+      lightTheme: "浅色主题",
+      open: "打开 I18n 开发工具设置",
+      panelPosition: "面板位置",
+      panelPositionControl: "I18n 开发工具面板位置",
+      right: "右侧",
+      theme: "主题",
+      themeControl: "I18n 开发工具主题",
+    },
+    trigger: {
+      close: "关闭 I18n 开发工具",
+      open: "打开 I18n 开发工具",
+    },
+  },
+  "zh-Hant": {
+    actions: {
+      compile: "編譯",
+      compileCompleted: "編譯完成。",
+      compileFailed: "編譯失敗。",
+      compiling: "正在編譯…",
+      enterFullscreen: "進入全螢幕",
+      exitFullscreen: "退出全螢幕",
+      extract: "擷取",
+      extractCompleted: "擷取完成。",
+      extractFailed: "擷取失敗。",
+      extracting: "正在擷取…",
+      label: "操作",
+    },
+    commandOutput: {
+      close: "關閉命令輸出",
+      collapse: "收合命令輸出",
+      expand: "展開命令輸出",
+      title: "命令輸出",
+    },
+    locale: {
+      controlLabel: "訊息目標語言",
+      label: "目標語言",
+      noTargetLocales: "尚未設定目標語言。",
+      sourceLabel: "來源語言",
+    },
+    messagePanel: {
+      additionalSourceLocations: (count) => `另外 ${count} 個來源位置`,
+      chooseCopyContent: "選擇要複製的內容",
+      collapseNamespace: (path) => `收合 ${path}`,
+      collapsePackage: (packageName) => `收合 ${packageName}`,
+      copied: "已複製",
+      copyDescriptor: "複製 Message Descriptor",
+      copyFailed: "複製失敗",
+      copyMessageId: "複製 Message ID",
+      copySource: "複製原文",
+      copySourceLocation: "複製來源位置",
+      copyTranslation: "複製目前譯文",
+      couldNotSave: "儲存失敗",
+      emptyCatalog: "尚無可用訊息。請先擷取專案訊息以產生目錄。",
+      emptyPackage: "這個套件中暫無訊息。",
+      expandNamespace: (path) => `展開 ${path}`,
+      expandPackage: (packageName) => `展開 ${packageName}`,
+      loading: (locale) => `正在載入 ${locale}…`,
+      messageCount: (count) => `共 ${count} 筆`,
+      messageEditor: "訊息編輯器",
+      messageNavigation: "訊息導覽",
+      missingCount: (count) => `缺漏 ${count} 筆`,
+      missingOnly: "僅顯示缺漏項目",
+      noMatches: "沒有符合目前篩選條件的訊息。",
+      noMissingMessages: "所有訊息皆已完成翻譯。",
+      noSearchResults: "沒有符合搜尋內容的訊息。",
+      reset: "重設",
+      retry: "重試",
+      save: "儲存",
+      saved: "已儲存",
+      saving: "正在儲存…",
+      search: "搜尋訊息",
+      selectMessage: "請選擇一則訊息以檢視和編輯譯文。",
+      source: "原文",
+      sourceLocations: "來源位置",
+      translation: "譯文",
+      translationFor: (id, locale) => `${id} 的 ${locale} 翻譯`,
+      unknownSourceLocation: "未知來源位置",
+    },
+    settings: {
+      auto: "自動",
+      autoTheme: "自動主題",
+      center: "置中",
+      dark: "深色",
+      darkTheme: "深色主題",
+      interfaceLanguage: "介面語言",
+      interfaceLanguageControl: "I18n 開發工具介面語言",
+      label: "I18n 開發工具設定",
+      left: "左側",
+      light: "淺色",
+      lightTheme: "淺色主題",
+      open: "開啟 I18n 開發工具設定",
+      panelPosition: "面板位置",
+      panelPositionControl: "I18n 開發工具面板位置",
+      right: "右側",
+      theme: "主題",
+      themeControl: "I18n 開發工具主題",
+    },
+    trigger: {
+      close: "關閉 I18n 開發工具",
+      open: "開啟 I18n 開發工具",
+    },
+  },
+}
+
+const defaultContextValue: {
+  locale: I18nDevtoolLocale
+  messages: DevtoolMessages
+} = {
+  locale: "en",
+  messages: messagesByLocale.en,
+}
+
+const DevtoolLocalizationContext = React.createContext(defaultContextValue)
+
+function matchDevtoolLocale(locale: string): I18nDevtoolLocale | undefined {
+  const normalizedLocale = locale.trim().replaceAll("_", "-").toLowerCase()
+
+  if (!normalizedLocale) {
+    return undefined
+  }
+
+  if (
+    normalizedLocale === "zh-hant" ||
+    normalizedLocale.startsWith("zh-hant-")
+  ) {
+    return "zh-Hant"
+  }
+
+  if (
+    normalizedLocale === "zh-tw" ||
+    normalizedLocale.startsWith("zh-tw-") ||
+    normalizedLocale === "zh-hk" ||
+    normalizedLocale.startsWith("zh-hk-") ||
+    normalizedLocale === "zh-mo" ||
+    normalizedLocale.startsWith("zh-mo-")
+  ) {
+    return "zh-Hant"
+  }
+
+  if (normalizedLocale === "zh" || normalizedLocale.startsWith("zh-")) {
+    return "zh-Hans"
+  }
+
+  const language = normalizedLocale.split("-")[0]
+
+  return I18N_DEVTOOL_LOCALES.find((candidate) => candidate === language)
+}
+
+function getNavigatorLocales() {
+  if (typeof navigator === "undefined") {
+    return []
+  }
+
+  if (navigator.languages.length > 0) {
+    return navigator.languages
+  }
+
+  return navigator.language ? [navigator.language] : []
+}
+
+export function resolveI18nDevtoolLocale(
+  locale?: string,
+  preferredLocales: readonly string[] = getNavigatorLocales()
+): I18nDevtoolLocale {
+  const candidates = locale ? [locale] : preferredLocales
+
+  for (const candidate of candidates) {
+    const matchedLocale = matchDevtoolLocale(candidate)
+
+    if (matchedLocale) {
+      return matchedLocale
+    }
+  }
+
+  return "en"
+}
+
+export function getI18nDevtoolMessages(locale: I18nDevtoolLocale) {
+  return messagesByLocale[locale]
+}
+
+export function DevtoolLocalizationProvider({
+  children,
+  locale,
+}: {
+  children: React.ReactNode
+  locale: I18nDevtoolLocale
+}) {
+  const value = React.useMemo(
+    () => ({
+      locale,
+      messages: messagesByLocale[locale],
+    }),
+    [locale]
+  )
+
+  return (
+    <DevtoolLocalizationContext.Provider value={value}>
+      {children}
+    </DevtoolLocalizationContext.Provider>
+  )
+}
+
+export function useDevtoolLocalization() {
+  return React.useContext(DevtoolLocalizationContext)
+}

packages/i18n/src/devtool/devtool-preferences.ts

@@ -0,0 +1,124 @@
+import * as React from "react"
+
+import {
+  I18N_DEVTOOL_LOCALES,
+  type I18nDevtoolLocale,
+} from "./devtool-localization"
+
+export type I18nDevtoolPanelPlacement = "center" | "left" | "right"
+export type I18nDevtoolTheme = "auto" | "dark" | "light"
+
+interface DevtoolPreferences {
+  interfaceLocale: I18nDevtoolLocale
+  panelPlacement: I18nDevtoolPanelPlacement
+  theme: I18nDevtoolTheme
+  version: 1
+}
+
+interface UseDevtoolPreferencesOptions {
+  defaultInterfaceLocale: I18nDevtoolLocale
+  defaultPanelPlacement: I18nDevtoolPanelPlacement
+  defaultTheme: I18nDevtoolTheme
+}
+
+const STORAGE_KEY = "i18n-devtool:preferences"
+
+function isPanelPlacement(value: unknown): value is I18nDevtoolPanelPlacement {
+  return value === "center" || value === "left" || value === "right"
+}
+
+function isTheme(value: unknown): value is I18nDevtoolTheme {
+  return value === "auto" || value === "dark" || value === "light"
+}
+
+function isInterfaceLocale(value: unknown): value is I18nDevtoolLocale {
+  return (
+    typeof value === "string" &&
+    I18N_DEVTOOL_LOCALES.includes(value as I18nDevtoolLocale)
+  )
+}
+
+function readStoredPreferences() {
+  if (typeof window === "undefined") {
+    return null
+  }
+
+  try {
+    const parsed = JSON.parse(
+      window.localStorage.getItem(STORAGE_KEY) ?? ""
+    ) as Partial<DevtoolPreferences>
+
+    if (
+      parsed.version !== 1 ||
+      !isPanelPlacement(parsed.panelPlacement) ||
+      !isTheme(parsed.theme)
+    ) {
+      return null
+    }
+
+    return {
+      interfaceLocale: isInterfaceLocale(parsed.interfaceLocale)
+        ? parsed.interfaceLocale
+        : undefined,
+      panelPlacement: parsed.panelPlacement,
+      theme: parsed.theme,
+      version: 1 as const,
+    }
+  } catch {
+    return null
+  }
+}
+
+export function useDevtoolPreferences({
+  defaultInterfaceLocale,
+  defaultPanelPlacement,
+  defaultTheme,
+}: UseDevtoolPreferencesOptions) {
+  const [preferences, setPreferences] = React.useState<DevtoolPreferences>(
+    () => {
+      const storedPreferences = readStoredPreferences()
+
+      return {
+        interfaceLocale:
+          storedPreferences?.interfaceLocale ?? defaultInterfaceLocale,
+        panelPlacement:
+          storedPreferences?.panelPlacement ?? defaultPanelPlacement,
+        theme: storedPreferences?.theme ?? defaultTheme,
+        version: 1,
+      }
+    }
+  )
+
+  React.useEffect(() => {
+    try {
+      window.localStorage.setItem(STORAGE_KEY, JSON.stringify(preferences))
+    } catch {
+      // Storage can be unavailable in privacy-restricted browser contexts.
+    }
+  }, [preferences])
+
+  const setPanelPlacement = React.useCallback(
+    (panelPlacement: I18nDevtoolPanelPlacement) => {
+      setPreferences((current) => ({ ...current, panelPlacement }))
+    },
+    []
+  )
+  const setTheme = React.useCallback((theme: I18nDevtoolTheme) => {
+    setPreferences((current) => ({ ...current, theme }))
+  }, [])
+  const setInterfaceLocale = React.useCallback(
+    (interfaceLocale: I18nDevtoolLocale) => {
+      setPreferences((current) => ({ ...current, interfaceLocale }))
+    },
+    []
+  )
+
+  return {
+    interfaceLocale: preferences.interfaceLocale,
+    panelPlacement: preferences.panelPlacement,
+    setInterfaceLocale,
+    setPanelPlacement,
+    setTheme,
+    theme: preferences.theme,
+  }
+}

packages/i18n/src/devtool/devtool-select.tsx

@@ -0,0 +1,38 @@
+import type * as React from "react"
+
+export interface DevtoolSelectOption {
+  disabled?: boolean
+  label: string
+  value: string
+}
+
+export interface DevtoolSelectProps extends Omit<
+  React.ComponentProps<"select">,
+  "children"
+> {
+  options: readonly DevtoolSelectOption[]
+}
+
+export function DevtoolSelect({
+  className,
+  options,
+  ...props
+}: DevtoolSelectProps) {
+  const resolvedClassName = ["i18n-devtool__select", className]
+    .filter(Boolean)
+    .join(" ")
+
+  return (
+    <select className={resolvedClassName} {...props}>
+      {options.map((option) => (
+        <option
+          key={option.value}
+          disabled={option.disabled}
+          value={option.value}
+        >
+          {option.label}
+        </option>
+      ))}
+    </select>
+  )
+}

packages/i18n/src/devtool/devtool-settings.tsx

@@ -0,0 +1,159 @@
+import * as React from "react"
+import {
+  AlignCenterIcon,
+  AlignLeftIcon,
+  AlignRightIcon,
+  SettingsIcon,
+} from "lucide-react"
+
+import type {
+  I18nDevtoolPanelPlacement,
+  I18nDevtoolTheme,
+} from "./devtool-preferences"
+import {
+  I18N_DEVTOOL_LOCALES,
+  type I18nDevtoolLocale,
+  useDevtoolLocalization,
+} from "./devtool-localization"
+import { DevtoolSelect } from "./devtool-select"
+import { DevtoolThemeSwitcher } from "./devtool-theme-switcher"
+
+export interface DevtoolSettingsProps {
+  interfaceLocale: I18nDevtoolLocale
+  onInterfaceLocaleChange: (locale: I18nDevtoolLocale) => void
+  onPanelPlacementChange: (placement: I18nDevtoolPanelPlacement) => void
+  onThemeChange: (theme: I18nDevtoolTheme) => void
+  panelPlacement: I18nDevtoolPanelPlacement
+  showPanelPlacement?: boolean
+  theme: I18nDevtoolTheme
+}
+
+const panelPlacements = [
+  { icon: AlignLeftIcon, label: "left", value: "left" },
+  { icon: AlignCenterIcon, label: "center", value: "center" },
+  { icon: AlignRightIcon, label: "right", value: "right" },
+] as const
+
+const interfaceLocaleLabels: Record<I18nDevtoolLocale, string> = {
+  de: "Deutsch",
+  en: "English",
+  es: "Español",
+  fr: "Français",
+  ja: "日本語",
+  ko: "한국어",
+  "zh-Hans": "简体中文",
+  "zh-Hant": "繁體中文",
+}
+
+export function DevtoolSettings({
+  interfaceLocale,
+  onInterfaceLocaleChange,
+  onPanelPlacementChange,
+  onThemeChange,
+  panelPlacement,
+  showPanelPlacement = true,
+  theme,
+}: DevtoolSettingsProps) {
+  const [isOpen, setOpen] = React.useState(false)
+  const rootRef = React.useRef<HTMLDivElement>(null)
+  const { messages } = useDevtoolLocalization()
+
+  React.useEffect(() => {
+    if (!isOpen) {
+      return
+    }
+
+    const handlePointerDown = (event: PointerEvent) => {
+      if (
+        event.target instanceof Node &&
+        !rootRef.current?.contains(event.target)
+      ) {
+        setOpen(false)
+      }
+    }
+    const handleKeyDown = (event: KeyboardEvent) => {
+      if (event.key !== "Escape") {
+        return
+      }
+
+      event.stopPropagation()
+      setOpen(false)
+    }
+
+    document.addEventListener("pointerdown", handlePointerDown, true)
+    document.addEventListener("keydown", handleKeyDown, true)
+
+    return () => {
+      document.removeEventListener("pointerdown", handlePointerDown, true)
+      document.removeEventListener("keydown", handleKeyDown, true)
+    }
+  }, [isOpen])
+
+  return (
+    <div className="i18n-devtool__settings" ref={rootRef}>
+      <button
+        type="button"
+        aria-controls="i18n-devtool-settings-panel"
+        aria-expanded={isOpen}
+        aria-label={messages.settings.open}
+        className="i18n-devtool__settings-trigger"
+        onClick={() => setOpen((current) => !current)}
+      >
+        <SettingsIcon aria-hidden="true" />
+      </button>
+
+      {isOpen && (
+        <section
+          id="i18n-devtool-settings-panel"
+          aria-label={messages.settings.label}
+          className="i18n-devtool__settings-panel"
+        >
+          <label className="i18n-devtool__settings-field">
+            <span>{messages.settings.interfaceLanguage}</span>
+            <DevtoolSelect
+              aria-label={messages.settings.interfaceLanguageControl}
+              options={I18N_DEVTOOL_LOCALES.map((locale) => ({
+                label: interfaceLocaleLabels[locale],
+                value: locale,
+              }))}
+              value={interfaceLocale}
+              onChange={(event) =>
+                onInterfaceLocaleChange(event.target.value as I18nDevtoolLocale)
+              }
+            />
+          </label>
+
+          <div className="i18n-devtool__settings-field">
+            <span>{messages.settings.theme}</span>
+            <DevtoolThemeSwitcher theme={theme} onThemeChange={onThemeChange} />
+          </div>
+
+          {showPanelPlacement && (
+            <div className="i18n-devtool__settings-field">
+              <span>{messages.settings.panelPosition}</span>
+              <div
+                aria-label={messages.settings.panelPositionControl}
+                className="i18n-devtool__settings-options"
+                role="group"
+              >
+                {panelPlacements.map(
+                  ({ icon: PlacementIcon, label, value }) => (
+                    <button
+                      key={value}
+                      type="button"
+                      aria-pressed={panelPlacement === value}
+                      onClick={() => onPanelPlacementChange(value)}
+                    >
+                      <PlacementIcon aria-hidden="true" />
+                      <span>{messages.settings[label]}</span>
+                    </button>
+                  )
+                )}
+              </div>
+            </div>
+          )}
+        </section>
+      )}
+    </div>
+  )
+}

packages/i18n/src/devtool/devtool-theme-switcher.tsx

@@ -0,0 +1,58 @@
+import { MonitorIcon, MoonIcon, SunIcon } from "lucide-react"
+
+import type { I18nDevtoolTheme } from "./devtool-preferences"
+import { useDevtoolLocalization } from "./devtool-localization"
+
+export interface DevtoolThemeSwitcherProps {
+  onThemeChange: (theme: I18nDevtoolTheme) => void
+  theme: I18nDevtoolTheme
+}
+
+const themes = [
+  {
+    accessibleLabel: "autoTheme",
+    icon: MonitorIcon,
+    label: "auto",
+    value: "auto",
+  },
+  {
+    accessibleLabel: "lightTheme",
+    icon: SunIcon,
+    label: "light",
+    value: "light",
+  },
+  {
+    accessibleLabel: "darkTheme",
+    icon: MoonIcon,
+    label: "dark",
+    value: "dark",
+  },
+] as const
+
+export function DevtoolThemeSwitcher({
+  onThemeChange,
+  theme,
+}: DevtoolThemeSwitcherProps) {
+  const { messages } = useDevtoolLocalization()
+
+  return (
+    <div
+      aria-label={messages.settings.themeControl}
+      className="i18n-devtool__theme-switcher"
+      role="group"
+    >
+      {themes.map(({ accessibleLabel, icon: ThemeIcon, label, value }) => (
+        <button
+          key={value}
+          type="button"
+          aria-label={messages.settings[accessibleLabel]}
+          aria-pressed={theme === value}
+          onClick={() => onThemeChange(value)}
+        >
+          <ThemeIcon aria-hidden="true" />
+          <span>{messages.settings[label]}</span>
+        </button>
+      ))}
+    </div>
+  )
+}

packages/i18n/src/devtool/http-message-repository.ts

@@ -1,5 +1,6 @@
 import type {
   DevtoolMessage,
+  DevtoolMessagePackage,
   MessageRepository,
   UpdateMessageInput,
 } from "./types"
@@ -38,6 +39,11 @@ export function createHttpMessageRepository(
 
       return readResponse<readonly DevtoolMessage[]>(response)
     },
+    async getPackages() {
+      const response = await fetch(`${normalizedBaseUrl}/packages`)
+
+      return readResponse<readonly DevtoolMessagePackage[]>(response)
+    },
     async updateMessage(input: UpdateMessageInput) {
       const response = await fetch(`${normalizedBaseUrl}/messages`, {
         body: JSON.stringify(input),

packages/i18n/src/devtool/i18n-devtool-trigger.tsx

@@ -0,0 +1,58 @@
+import { LanguagesIcon } from "lucide-react"
+
+import { useDevtoolLocalization } from "./devtool-localization"
+import { useFloatingTrigger } from "./use-floating-trigger"
+
+export interface I18nDevtoolTriggerProps {
+  controls?: string
+  open?: boolean
+  onOpen: () => void
+  storageKey?: string
+}
+
+export function I18nDevtoolTrigger({
+  controls = "i18n-devtool-panel",
+  open = false,
+  onOpen,
+  storageKey,
+}: I18nDevtoolTriggerProps) {
+  const floatingTrigger = useFloatingTrigger({ storageKey })
+  const { messages } = useDevtoolLocalization()
+
+  return (
+    <button
+      type="button"
+      aria-controls={controls}
+      aria-expanded={open}
+      aria-label={messages.trigger.open}
+      className="i18n-devtool__trigger"
+      data-collapsed={floatingTrigger.isCollapsed || undefined}
+      data-dock={floatingTrigger.dockEdge}
+      data-dragging={floatingTrigger.isDragging || undefined}
+      hidden={open}
+      ref={floatingTrigger.triggerRef}
+      style={floatingTrigger.style}
+      onBlur={floatingTrigger.handleBlur}
+      onClick={(event) => {
+        if (floatingTrigger.handleClick()) {
+          event.preventDefault()
+          return
+        }
+
+        onOpen()
+      }}
+      onFocus={floatingTrigger.handleFocus}
+      onPointerCancel={floatingTrigger.handlePointerCancel}
+      onPointerDown={floatingTrigger.handlePointerDown}
+      onPointerEnter={floatingTrigger.handlePointerEnter}
+      onPointerLeave={floatingTrigger.handlePointerLeave}
+      onPointerMove={floatingTrigger.handlePointerMove}
+      onPointerUp={floatingTrigger.handlePointerUp}
+    >
+      <span className="i18n-devtool__trigger-icon" aria-hidden="true">
+        <LanguagesIcon />
+      </span>
+      <span className="i18n-devtool__trigger-label">i18n</span>
+    </button>
+  )
+}

packages/i18n/src/devtool/i18n-devtool.css

@@ -5,6 +5,8 @@
   --i18n-devtool-muted: #71717a;
   --i18n-devtool-panel: #fff;
   --i18n-devtool-primary: #2563eb;
+  --i18n-devtool-control-hover: rgb(15 23 42 / 6%);
+  --i18n-devtool-control-pressed: rgb(15 23 42 / 11%);
   position: fixed;
   z-index: 2147483647;
   color: var(--i18n-devtool-foreground);
@@ -26,6 +28,8 @@
   --i18n-devtool-muted: #a1a1aa;
   --i18n-devtool-panel: #181a20;
   --i18n-devtool-primary: #60a5fa;
+  --i18n-devtool-control-hover: rgb(255 255 255 / 8%);
+  --i18n-devtool-control-pressed: rgb(255 255 255 / 14%);
   color-scheme: dark;
 }
 
@@ -46,10 +50,13 @@
   right: 1rem;
   bottom: 1rem;
   display: inline-flex;
-  height: 2.75rem;
+  width: max-content;
+  height: 3rem;
   align-items: center;
-  gap: 0.45rem;
-  padding: 0 0.85rem;
+  gap: 0.5rem;
+  justify-content: center;
+  overflow: visible;
+  padding: 0 0.5rem;
   border: 1px solid var(--i18n-devtool-border);
   border-radius: 999px;
   background: var(--i18n-devtool-panel);
@@ -57,28 +64,169 @@
   cursor: pointer;
   font-size: 0.8125rem;
   font-weight: 650;
+  touch-action: none;
+  transition:
+    gap 140ms ease,
+    border-radius 140ms ease,
+    background-color 120ms ease,
+    box-shadow 140ms ease,
+    transform 160ms ease;
+  user-select: none;
 }
 
-.i18n-devtool__trigger span:first-child {
+.i18n-devtool__trigger::before {
+  position: absolute;
+  display: none;
+  content: "";
+}
+
+.i18n-devtool__trigger[hidden] {
+  display: none;
+}
+
+.i18n-devtool__trigger:focus-visible {
+  outline: 2px solid var(--i18n-devtool-primary);
+  outline-offset: 2px;
+}
+
+.i18n-devtool__trigger[data-dragging] {
+  cursor: grabbing;
+  box-shadow: 0 16px 36px rgb(0 0 0 / 24%);
+}
+
+.i18n-devtool__trigger:hover {
+  background: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary) 7%,
+    var(--i18n-devtool-panel)
+  );
+}
+
+.i18n-devtool__trigger:active {
+  background: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary) 12%,
+    var(--i18n-devtool-panel)
+  );
+}
+
+.i18n-devtool__trigger[data-collapsed] {
+  gap: 0;
+}
+
+.i18n-devtool__trigger[data-dock="left"] {
+  border-top-left-radius: 0;
+  border-bottom-left-radius: 0;
+}
+
+.i18n-devtool__trigger[data-dock="right"] {
+  border-top-right-radius: 0;
+  border-bottom-right-radius: 0;
+}
+
+.i18n-devtool__trigger[data-dock="top"] {
+  border-top-left-radius: 0;
+  border-top-right-radius: 0;
+}
+
+.i18n-devtool__trigger[data-dock="bottom"] {
+  border-bottom-right-radius: 0;
+  border-bottom-left-radius: 0;
+}
+
+.i18n-devtool__trigger[data-dock]:not([data-collapsed]) {
+  border-radius: 999px;
+}
+
+.i18n-devtool__trigger[data-dock="left"]:not([data-collapsed]) {
+  transform: translateX(0.5rem);
+}
+
+.i18n-devtool__trigger[data-dock="right"]:not([data-collapsed]) {
+  transform: translateX(-0.5rem);
+}
+
+.i18n-devtool__trigger[data-dock="top"]:not([data-collapsed]) {
+  transform: translateY(0.5rem);
+}
+
+.i18n-devtool__trigger[data-dock="bottom"]:not([data-collapsed]) {
+  transform: translateY(-0.5rem);
+}
+
+.i18n-devtool__trigger[data-dock="left"]:not([data-collapsed])::before {
+  inset-block: 0;
+  right: 100%;
+  display: block;
+  width: 0.5rem;
+}
+
+.i18n-devtool__trigger[data-dock="right"]:not([data-collapsed])::before {
+  inset-block: 0;
+  left: 100%;
+  display: block;
+  width: 0.5rem;
+}
+
+.i18n-devtool__trigger[data-dock="top"]:not([data-collapsed])::before {
+  bottom: 100%;
+  inset-inline: 0;
+  display: block;
+  height: 0.5rem;
+}
+
+.i18n-devtool__trigger[data-dock="bottom"]:not([data-collapsed])::before {
+  top: 100%;
+  inset-inline: 0;
+  display: block;
+  height: 0.5rem;
+}
+
+.i18n-devtool__trigger-icon {
   display: grid;
-  width: 1.5rem;
-  height: 1.5rem;
+  width: 2rem;
+  height: 2rem;
+  flex: 0 0 2rem;
   place-items: center;
-  border-radius: 0.4rem;
+  border-radius: 50%;
   background: var(--i18n-devtool-primary);
   color: white;
-  font-size: 0.75rem;
+}
+
+.i18n-devtool__trigger-icon svg {
+  display: block;
+  width: 1.25rem;
+  height: 1.25rem;
+  stroke-width: 2.25;
+}
+
+.i18n-devtool__trigger-label {
+  width: 2.5rem;
+  max-width: 2.5rem;
+  overflow: hidden;
+  opacity: 1;
+  transform: translateX(0);
+  transition:
+    max-width 140ms ease,
+    opacity 100ms ease,
+    transform 140ms ease;
+  white-space: nowrap;
+}
+
+.i18n-devtool__trigger[data-collapsed] .i18n-devtool__trigger-label {
+  max-width: 0;
+  opacity: 0;
+  transform: translateX(-0.35rem);
 }
 
 .i18n-devtool__panel {
   position: fixed;
   top: 1rem;
-  right: 1rem;
   bottom: 1rem;
-  display: grid;
-  width: min(54rem, calc(100vw - 2rem));
+  display: flex;
+  width: min(80rem, calc(100vw - 2rem));
   overflow: hidden;
-  grid-template-rows: auto auto minmax(0, 1fr);
+  flex-direction: column;
   border: 1px solid var(--i18n-devtool-border);
   border-radius: 1rem;
   outline: none;
@@ -86,6 +234,22 @@
   box-shadow: 0 24px 70px rgb(0 0 0 / 28%);
 }
 
+.i18n-devtool__panel[data-placement="left"] {
+  right: auto;
+  left: 1rem;
+}
+
+.i18n-devtool__panel[data-placement="center"] {
+  right: auto;
+  left: 50%;
+  transform: translateX(-50%);
+}
+
+.i18n-devtool__panel[data-placement="right"] {
+  right: 1rem;
+  left: auto;
+}
+
 .i18n-devtool[data-mode="standalone"] {
   inset: 0;
 }
@@ -96,9 +260,21 @@
   border: 0;
   border-radius: 0;
   box-shadow: none;
+  transform: none;
+}
+
+.i18n-devtool__panel[data-fullscreen] {
+  inset: 0;
+  width: 100vw;
+  border: 0;
+  border-radius: 0;
+  box-shadow: none;
+  transform: none;
 }
 
 .i18n-devtool__header {
+  position: relative;
+  z-index: 3;
   display: flex;
   align-items: center;
   justify-content: space-between;
@@ -129,58 +305,347 @@
 .i18n-devtool__controls {
   display: flex;
   align-items: end;
-  gap: 0.4rem;
+  gap: 1rem;
 }
 
-.i18n-devtool__controls label {
+.i18n-devtool__locale-field,
+.i18n-devtool__source-locale-field,
+.i18n-devtool__action-field {
   display: grid;
   gap: 0.2rem;
 }
 
-.i18n-devtool__controls label span {
+.i18n-devtool__action-field {
+  min-width: 0;
+  margin: 0;
+  padding: 0;
+  border: 0;
+}
+
+.i18n-devtool__locale-field > span,
+.i18n-devtool__source-locale-field > span,
+.i18n-devtool__action-field > legend {
+  padding: 0;
   color: var(--i18n-devtool-muted);
   font-size: 0.625rem;
   font-weight: 650;
   text-transform: uppercase;
 }
 
-.i18n-devtool__controls select,
-.i18n-devtool__controls button {
+.i18n-devtool__source-locale {
+  display: inline-flex;
+  min-width: 7rem;
+  height: 2.25rem;
+  align-items: center;
+  padding: 0 0.7rem;
+  border: 1px solid var(--i18n-devtool-border);
+  border-radius: 0.55rem;
+  background: color-mix(
+    in srgb,
+    var(--i18n-devtool-panel) 82%,
+    var(--i18n-devtool-muted) 18%
+  );
+  color: var(--i18n-devtool-muted);
+  white-space: nowrap;
+}
+
+.i18n-devtool__action-controls {
+  display: flex;
+  align-items: center;
+  gap: 0.4rem;
+}
+
+.i18n-devtool__action-button {
+  display: grid;
+  place-items: center;
+}
+
+.i18n-devtool__action-button > span {
+  grid-area: 1 / 1;
+}
+
+.i18n-devtool__action-label-measure {
+  visibility: hidden;
+}
+
+.i18n-devtool__select,
+.i18n-devtool__action-controls > button,
+.i18n-devtool__settings-trigger {
   height: 2.25rem;
   padding: 0 0.7rem;
   border: 1px solid var(--i18n-devtool-border);
   border-radius: 0.55rem;
+  outline: none;
   background: var(--i18n-devtool-panel);
+  transition:
+    background-color 120ms ease,
+    border-color 120ms ease,
+    box-shadow 120ms ease,
+    transform 80ms ease;
 }
 
-.i18n-devtool__controls button {
+.i18n-devtool__select {
+  color: var(--i18n-devtool-foreground);
   cursor: pointer;
 }
 
-.i18n-devtool__controls button:disabled {
+.i18n-devtool__locale-select {
+  min-width: 9rem;
+}
+
+.i18n-devtool__action-controls > button,
+.i18n-devtool__settings-trigger {
+  cursor: pointer;
+}
+
+.i18n-devtool__select:hover,
+.i18n-devtool__action-controls > button:hover:not(:disabled),
+.i18n-devtool__settings-trigger:hover {
+  background: var(--i18n-devtool-control-hover);
+}
+
+.i18n-devtool__select:focus-visible,
+.i18n-devtool__action-controls > button:focus-visible,
+.i18n-devtool__settings-trigger:focus-visible {
+  border-color: var(--i18n-devtool-primary);
+  box-shadow: 0 0 0 3px
+    color-mix(in srgb, var(--i18n-devtool-primary) 18%, transparent);
+}
+
+.i18n-devtool__action-controls > button:active:not(:disabled),
+.i18n-devtool__settings-trigger:active {
+  background: var(--i18n-devtool-control-pressed);
+  transform: translateY(1px);
+}
+
+.i18n-devtool__action-controls > button:disabled {
   cursor: default;
   opacity: 0.45;
 }
 
-.i18n-devtool__controls .i18n-devtool__close {
+.i18n-devtool__controls .i18n-devtool__close,
+.i18n-devtool__controls .i18n-devtool__fullscreen {
+  display: inline-grid;
   width: 2.25rem;
+  place-items: center;
   padding: 0;
-  font-size: 1.25rem;
+}
+
+.i18n-devtool__close svg,
+.i18n-devtool__fullscreen svg {
+  width: 1rem;
+  height: 1rem;
+}
+
+.i18n-devtool__fullscreen[aria-pressed="true"] {
+  border-color: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary) 45%,
+    var(--i18n-devtool-border)
+  );
+  background: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary) 12%,
+    var(--i18n-devtool-panel)
+  );
+  color: var(--i18n-devtool-primary);
+}
+
+.i18n-devtool__theme-switcher {
+  display: grid;
+  grid-template-columns: repeat(3, minmax(0, 1fr));
+  gap: 0.25rem;
+  padding: 0.25rem;
+  border: 1px solid var(--i18n-devtool-border);
+  border-radius: 0.625rem;
+  background: var(--i18n-devtool-panel);
+}
+
+.i18n-devtool__theme-switcher button {
+  display: inline-flex;
+  min-width: 0;
+  height: 2.25rem;
+  align-items: center;
+  justify-content: center;
+  gap: 0.375rem;
+  padding: 0 0.5rem;
+  border: 0;
+  border-radius: 0.375rem;
+  outline: none;
+  background: transparent;
+  color: var(--i18n-devtool-muted);
+  cursor: pointer;
+  transition:
+    color 120ms ease,
+    background-color 120ms ease,
+    transform 80ms ease;
+}
+
+.i18n-devtool__theme-switcher button:hover {
+  background: var(--i18n-devtool-control-hover);
+  color: var(--i18n-devtool-foreground);
+}
+
+.i18n-devtool__theme-switcher button:active {
+  transform: scale(0.94);
+}
+
+.i18n-devtool__theme-switcher button:focus-visible {
+  box-shadow: 0 0 0 2px var(--i18n-devtool-primary);
+}
+
+.i18n-devtool__theme-switcher button[aria-pressed="true"] {
+  background: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary) 14%,
+    var(--i18n-devtool-panel)
+  );
+  color: var(--i18n-devtool-primary);
+}
+
+.i18n-devtool__theme-switcher svg {
+  width: 1rem;
+  height: 1rem;
+  flex: 0 0 1rem;
+}
+
+.i18n-devtool__theme-switcher span {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.i18n-devtool__settings {
+  position: relative;
+  align-self: end;
+}
+
+.i18n-devtool__settings-trigger {
+  display: inline-grid;
+  width: 2.25rem;
+  place-items: center;
+  padding: 0;
+}
+
+.i18n-devtool__settings-trigger svg {
+  width: 1rem;
+  height: 1rem;
+}
+
+.i18n-devtool__settings-trigger[aria-expanded="true"] {
+  border-color: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary) 45%,
+    var(--i18n-devtool-border)
+  );
+  background: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary) 12%,
+    var(--i18n-devtool-panel)
+  );
+  color: var(--i18n-devtool-primary);
+}
+
+.i18n-devtool__settings-panel {
+  position: absolute;
+  z-index: 5;
+  top: calc(100% + 0.5rem);
+  right: 0;
+  display: grid;
+  width: 20rem;
+  max-width: calc(100vw - 3rem);
+  gap: 1rem;
+  padding: 1rem;
+  border: 1px solid var(--i18n-devtool-border);
+  border-radius: 0.875rem;
+  background: var(--i18n-devtool-panel);
+  box-shadow: 0 18px 48px rgb(0 0 0 / 22%);
+}
+
+.i18n-devtool__settings-field {
+  display: grid;
+  gap: 0.45rem;
+}
+
+.i18n-devtool__settings-field > span {
+  color: var(--i18n-devtool-muted);
+  font-size: 0.675rem;
+  font-weight: 700;
+  letter-spacing: 0.04em;
+  text-transform: uppercase;
+}
+
+.i18n-devtool__settings-panel .i18n-devtool__select {
+  width: 100%;
+  height: 2.75rem;
+  border-radius: 0.625rem;
+}
+
+.i18n-devtool__settings-options {
+  display: grid;
+  grid-template-columns: repeat(3, minmax(0, 1fr));
+  gap: 0.25rem;
+  padding: 0.25rem;
+  border: 1px solid var(--i18n-devtool-border);
+  border-radius: 0.625rem;
+  background: var(--i18n-devtool-panel);
+}
+
+.i18n-devtool__settings-options button {
+  display: inline-flex;
+  min-width: 0;
+  height: 2.25rem;
+  align-items: center;
+  justify-content: center;
+  gap: 0.375rem;
+  padding: 0 0.5rem;
+  border: 0;
+  border-radius: 0.375rem;
+  outline: none;
+  background: transparent;
+  color: var(--i18n-devtool-muted);
+  cursor: pointer;
+  transition:
+    color 120ms ease,
+    background-color 120ms ease,
+    transform 80ms ease;
+}
+
+.i18n-devtool__settings-options button:hover {
+  background: var(--i18n-devtool-control-hover);
+  color: var(--i18n-devtool-foreground);
+}
+
+.i18n-devtool__settings-options button:active {
+  transform: scale(0.97);
+}
+
+.i18n-devtool__settings-options button:focus-visible {
+  box-shadow: 0 0 0 2px var(--i18n-devtool-primary);
+}
+
+.i18n-devtool__settings-options button[aria-pressed="true"] {
+  background: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary) 14%,
+    var(--i18n-devtool-panel)
+  );
+  color: var(--i18n-devtool-primary);
+}
+
+.i18n-devtool__settings-options svg {
+  width: 1rem;
+  height: 1rem;
+  flex: 0 0 1rem;
 }
 
 .i18n-devtool__output {
-  max-height: 10rem;
-  overflow: auto;
-  margin: 0;
-  padding: 0.65rem 1rem;
   border-bottom: 1px solid var(--i18n-devtool-border);
   background: color-mix(
     in srgb,
     var(--i18n-devtool-primary) 10%,
     var(--i18n-devtool-panel)
   );
-  font-size: 0.72rem;
-  white-space: pre-wrap;
 }
 
 .i18n-devtool__output[data-error] {
@@ -188,15 +653,100 @@
   color: #ef4444;
 }
 
-.i18n-devtool__main {
-  min-height: 0;
+.i18n-devtool__output-header {
+  display: flex;
+  min-height: 2.25rem;
+  align-items: center;
+  justify-content: space-between;
+  gap: 1rem;
+  padding: 0.25rem 0.5rem 0.25rem 1rem;
+  color: var(--i18n-devtool-muted);
+  font-size: 0.7rem;
+  font-weight: 650;
+  text-transform: uppercase;
+}
+
+.i18n-devtool__output-header > span,
+.i18n-devtool__output-header > div {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.375rem;
+}
+
+.i18n-devtool__output-header button {
+  display: inline-grid;
+  width: 1.75rem;
+  height: 1.75rem;
+  place-items: center;
+  padding: 0;
+  border: 0;
+  border-radius: 0.375rem;
+  outline: none;
+  background: transparent;
+  color: inherit;
+  cursor: pointer;
+}
+
+.i18n-devtool__output-header button:hover {
+  background: var(--i18n-devtool-control-hover);
+  color: var(--i18n-devtool-foreground);
+}
+
+.i18n-devtool__output-header button:active {
+  background: var(--i18n-devtool-control-pressed);
+}
+
+.i18n-devtool__output-header button:focus-visible {
+  box-shadow: 0 0 0 2px var(--i18n-devtool-primary);
+}
+
+.i18n-devtool__output-header svg {
+  width: 0.875rem;
+  height: 0.875rem;
+}
+
+.i18n-devtool__output pre {
+  max-height: 10rem;
   overflow: auto;
+  margin: 0;
+  padding: 0.65rem 1rem;
+  border-top: 1px solid var(--i18n-devtool-border);
+  font:
+    0.72rem/1.5 ui-monospace,
+    SFMono-Regular,
+    Menlo,
+    Monaco,
+    Consolas,
+    monospace;
+  white-space: pre-wrap;
+}
+
+.i18n-devtool__main {
+  flex: 1;
+  min-height: 0;
+  overflow: hidden;
+}
+
+.i18n-devtool__empty {
+  display: grid;
+  min-height: 100%;
+  place-items: center;
+  padding: 2rem;
+  color: var(--i18n-devtool-muted);
+  text-align: center;
 }
 
 .i18n-devtool .i18n-message-panel {
   --i18n-border: var(--i18n-devtool-border);
   --i18n-muted: var(--i18n-devtool-muted);
   --i18n-surface: var(--i18n-devtool-panel);
+  height: 100%;
+}
+
+.i18n-devtool .i18n-message-workspace {
+  height: 0;
+  min-height: 0;
+  overflow: hidden;
 }
 
 .i18n-devtool .i18n-message-panel__toolbar {
@@ -209,15 +759,116 @@
 
 .i18n-devtool .i18n-message-panel input[type="search"],
 .i18n-devtool .i18n-message-panel textarea,
-.i18n-devtool .i18n-message-panel button {
+.i18n-devtool .i18n-message-panel button:not(.i18n-message-copy__trigger) {
   background: var(--i18n-devtool-panel);
 }
 
+.i18n-devtool .i18n-message-panel button.i18n-message-copy__trigger,
+.i18n-devtool
+  .i18n-message-panel
+  button.i18n-message-copy__trigger:hover:not(:disabled),
+.i18n-devtool
+  .i18n-message-panel
+  button.i18n-message-copy__trigger[aria-expanded="true"] {
+  background: transparent;
+}
+
+.i18n-devtool
+  .i18n-message-panel
+  .i18n-message-copy__menu
+  > button:hover:not(:disabled),
+.i18n-devtool
+  .i18n-message-panel
+  .i18n-message-copy__menu
+  > button:focus-visible {
+  background: var(--i18n-devtool-control-hover);
+}
+
+.i18n-devtool .i18n-message-copy__feedback {
+  background: var(--i18n-devtool-panel);
+}
+
+.i18n-devtool .i18n-message-panel .i18n-message-navigation button {
+  background: transparent;
+}
+
+@supports (appearance: base-select) {
+  .i18n-devtool__select,
+  .i18n-devtool__select::picker(select) {
+    appearance: base-select;
+  }
+
+  .i18n-devtool__select {
+    align-items: center;
+    gap: 0.5rem;
+  }
+
+  .i18n-devtool__select::picker-icon {
+    width: 1rem;
+    height: 1rem;
+    flex: 0 0 1rem;
+    background: currentColor;
+    content: "";
+    mask: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='black' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Cpath d='m6 9 6 6 6-6'/%3E%3C/svg%3E")
+      center / contain no-repeat;
+    transition: rotate 140ms ease;
+  }
+
+  .i18n-devtool__select:open::picker-icon {
+    rotate: 180deg;
+  }
+
+  .i18n-devtool__select::picker(select) {
+    margin-block-start: 0.25rem;
+    padding: 0.25rem;
+    border: 1px solid var(--i18n-devtool-border);
+    border-radius: 0.625rem;
+    background: var(--i18n-devtool-panel);
+    box-shadow: 0 12px 32px rgb(0 0 0 / 20%);
+    color: var(--i18n-devtool-foreground);
+  }
+
+  .i18n-devtool__select option {
+    min-height: 2.25rem;
+    align-items: center;
+    gap: 0.5rem;
+    padding: 0.4rem 0.625rem;
+    border: 0;
+    border-radius: 0.4rem;
+    background: transparent;
+    color: inherit;
+  }
+
+  .i18n-devtool__select option:hover,
+  .i18n-devtool__select option:focus {
+    background: var(--i18n-devtool-control-hover);
+  }
+
+  .i18n-devtool__select option:checked {
+    background: color-mix(
+      in srgb,
+      var(--i18n-devtool-primary) 12%,
+      var(--i18n-devtool-panel)
+    );
+    color: var(--i18n-devtool-primary);
+  }
+
+  .i18n-devtool__select option::checkmark {
+    width: 1rem;
+    height: 1rem;
+    background: currentColor;
+    content: "";
+    mask: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='black' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Cpath d='M20 6 9 17l-5-5'/%3E%3C/svg%3E")
+      center / contain no-repeat;
+  }
+}
+
 @media (max-width: 42rem) {
   .i18n-devtool__panel {
     inset: 0;
     width: 100vw;
     border-radius: 0;
+    transform: none;
   }
 
   .i18n-devtool__header {
@@ -229,8 +880,13 @@
     flex-wrap: wrap;
   }
 
-  .i18n-devtool__controls label {
+  .i18n-devtool__locale-field,
+  .i18n-devtool__source-locale-field {
     min-width: 10rem;
     flex: 1;
   }
+
+  .i18n-devtool__action-field {
+    flex: none;
+  }
 }

packages/i18n/src/devtool/i18n-devtool.test.tsx

@@ -1,17 +1,159 @@
 // @vitest-environment jsdom
 
 import * as React from "react"
-import { act, render, screen, waitFor } from "@testing-library/react"
-import { afterEach, describe, expect, it } from "vitest"
+import {
+  act,
+  cleanup,
+  fireEvent,
+  render,
+  screen,
+  waitFor,
+} from "@testing-library/react"
+import { afterEach, describe, expect, it, vi } from "vitest"
 
 import { I18nProvider } from "../runtime"
-import { I18nDevtool } from "./i18n-devtool"
+import { I18nDevtool, type I18nDevtoolProps } from "./i18n-devtool"
 
 afterEach(() => {
+  cleanup()
   document.documentElement.classList.remove("ui:dark")
+  window.localStorage.clear()
+  vi.restoreAllMocks()
+  vi.unstubAllGlobals()
 })
 
+function renderDevtool(props?: I18nDevtoolProps) {
+  return render(
+    <I18nProvider catalogs={{ en: {} }} locale="en" locales={["en"]}>
+      <I18nDevtool dark="ui:dark" {...props} />
+    </I18nProvider>
+  )
+}
+
+function mockDevtoolApi(output = "Done") {
+  vi.stubGlobal(
+    "fetch",
+    vi.fn(async (input: RequestInfo | URL) => {
+      const url = String(input)
+
+      return new Response(
+        JSON.stringify(url.includes("/actions/") ? { output } : []),
+        {
+          headers: { "content-type": "application/json" },
+          status: 200,
+        }
+      )
+    })
+  )
+}
+
+function prepareTriggerForDragging(
+  trigger: HTMLElement,
+  { left, top }: { left: number; top: number }
+) {
+  vi.spyOn(trigger, "getBoundingClientRect").mockReturnValue({
+    bottom: top + 44,
+    height: 44,
+    left,
+    right: left + 80,
+    top,
+    width: 80,
+    x: left,
+    y: top,
+    toJSON: () => ({}),
+  })
+  trigger.setPointerCapture = vi.fn()
+  trigger.hasPointerCapture = vi.fn(() => false)
+}
+
 describe("I18nDevtool", () => {
+  it("uses an explicitly configured control-panel locale", async () => {
+    mockDevtoolApi()
+    renderDevtool({ defaultOpen: true, locale: "zh-Hans" })
+
+    const dialog = await screen.findByRole("dialog")
+    const devtool = dialog.closest<HTMLElement>('[data-slot="i18n-devtool"]')
+
+    expect(devtool?.getAttribute("lang")).toBe("zh-Hans")
+    expect(screen.getByText("操作")).toBeTruthy()
+    expect(screen.getByRole("button", { name: "提取" })).toBeTruthy()
+    expect(screen.getByRole("button", { name: "编译" })).toBeTruthy()
+    expect(screen.getByPlaceholderText("搜索消息")).toBeTruthy()
+    expect(
+      await screen.findByText("尚无可用消息。请先提取项目消息以生成目录。")
+    ).toBeTruthy()
+  })
+
+  it("detects the first supported navigator locale", async () => {
+    vi.spyOn(window.navigator, "languages", "get").mockReturnValue([
+      "pt-BR",
+      "ja-JP",
+      "en-US",
+    ])
+    renderDevtool()
+
+    const trigger = await screen.findByRole("button", {
+      name: "I18n Devtool を開く",
+    })
+    const devtool = trigger.closest<HTMLElement>('[data-slot="i18n-devtool"]')
+
+    expect(devtool?.getAttribute("lang")).toBe("ja")
+  })
+
+  it("falls back to English when an explicit locale is unsupported", async () => {
+    vi.spyOn(window.navigator, "languages", "get").mockReturnValue(["zh-CN"])
+    renderDevtool({ locale: "pt-BR" })
+
+    const trigger = await screen.findByRole("button", {
+      name: "Open I18n Devtool",
+    })
+    const devtool = trigger.closest<HTMLElement>('[data-slot="i18n-devtool"]')
+
+    expect(devtool?.getAttribute("lang")).toBe("en")
+  })
+
+  it("includes the project source locale as a customization target", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async (input: RequestInfo | URL) => {
+        const url = String(input)
+        const body = url.endsWith("/project")
+          ? { locales: ["en", "zh-Hans"], sourceLocale: "en" }
+          : []
+
+        return new Response(JSON.stringify(body), {
+          headers: { "content-type": "application/json" },
+          status: 200,
+        })
+      })
+    )
+
+    render(
+      <I18nProvider
+        catalogs={{ en: {}, "zh-Hans": {} }}
+        locale="en"
+        locales={[
+          { label: "English", locale: "en" },
+          { label: "简体中文", locale: "zh-Hans" },
+        ]}
+      >
+        <I18nDevtool dark="ui:dark" defaultOpen />
+      </I18nProvider>
+    )
+
+    const localeSelect = await screen.findByRole("combobox", {
+      name: "Target message locale",
+    })
+
+    await waitFor(() => {
+      expect(localeSelect).toHaveProperty("value", "en")
+    })
+    expect(screen.getByText("Source locale")).toBeTruthy()
+    expect(screen.getByText("English", { selector: "output" })).toBeTruthy()
+    expect(screen.getByRole("option", { name: "English" })).toBeTruthy()
+    expect(screen.getByRole("option", { name: "简体中文" })).toBeTruthy()
+  })
+
   it("tracks the configured document dark-mode class", async () => {
     render(
       <I18nProvider
@@ -39,4 +181,445 @@ describe("I18nDevtool", () => {
       expect(devtool?.hasAttribute("data-dark")).toBe(true)
     })
   })
+
+  it("preserves panel state while Activity hides the floating panel", async () => {
+    mockDevtoolApi()
+    renderDevtool()
+
+    fireEvent.click(
+      await screen.findByRole("button", {
+        name: "Open I18n Devtool",
+      })
+    )
+    fireEvent.change(await screen.findByPlaceholderText("Search messages"), {
+      target: { value: "navigation" },
+    })
+    fireEvent.click(screen.getByRole("button", { name: "Close I18n Devtool" }))
+    fireEvent.click(
+      await screen.findByRole("button", {
+        name: "Open I18n Devtool",
+      })
+    )
+
+    expect(screen.getByPlaceholderText("Search messages")).toHaveProperty(
+      "value",
+      "navigation"
+    )
+  })
+
+  it("toggles full screen and lets Escape restore the floating panel", async () => {
+    mockDevtoolApi()
+    renderDevtool({ defaultOpen: true })
+
+    const dialog = await screen.findByRole("dialog")
+    const enterFullscreen = screen.getByRole("button", {
+      name: "Enter full screen",
+    })
+
+    expect(dialog.hasAttribute("data-fullscreen")).toBe(false)
+    fireEvent.click(enterFullscreen)
+    expect(dialog.hasAttribute("data-fullscreen")).toBe(true)
+    expect(
+      screen.getByRole("button", { name: "Exit full screen" })
+    ).toBeTruthy()
+
+    fireEvent.keyDown(window, { key: "Escape" })
+    expect(dialog.hasAttribute("data-fullscreen")).toBe(false)
+    expect(screen.getByRole("dialog")).toBeTruthy()
+  })
+
+  it("expands a docked trigger while it is hovered or focused", async () => {
+    renderDevtool()
+    const trigger = await screen.findByRole("button", {
+      name: "Open I18n Devtool",
+    })
+
+    expect(trigger.getAttribute("data-dock")).toBe("right")
+    expect(trigger.hasAttribute("data-collapsed")).toBe(true)
+
+    fireEvent.pointerEnter(trigger)
+    expect(trigger.hasAttribute("data-collapsed")).toBe(false)
+
+    fireEvent.pointerLeave(trigger)
+    expect(trigger.hasAttribute("data-collapsed")).toBe(true)
+
+    fireEvent.focus(trigger)
+    expect(trigger.hasAttribute("data-collapsed")).toBe(false)
+  })
+
+  it("keeps a docked trigger stationary when clicked and collapses it after closing", async () => {
+    renderDevtool()
+    const trigger = await screen.findByRole("button", {
+      name: "Open I18n Devtool",
+    })
+
+    prepareTriggerForDragging(trigger, { left: 944, top: 700 })
+    fireEvent.pointerEnter(trigger)
+    fireEvent.focus(trigger)
+    fireEvent.pointerDown(trigger, {
+      button: 0,
+      clientX: 964,
+      clientY: 720,
+      pointerId: 1,
+    })
+
+    expect(trigger.getAttribute("data-dock")).toBe("right")
+    expect(trigger.hasAttribute("data-dragging")).toBe(false)
+
+    fireEvent.pointerUp(trigger, {
+      clientX: 964,
+      clientY: 720,
+      pointerId: 1,
+    })
+    fireEvent.click(trigger)
+
+    fireEvent.click(
+      await screen.findByRole("button", { name: "Close I18n Devtool" })
+    )
+
+    expect(trigger.getAttribute("data-dock")).toBe("right")
+    expect(trigger.hasAttribute("data-collapsed")).toBe(true)
+  })
+
+  it("only enters dragging state after a long press and does not open on release", async () => {
+    renderDevtool()
+    const trigger = await screen.findByRole("button", {
+      name: "Open I18n Devtool",
+    })
+
+    prepareTriggerForDragging(trigger, { left: 944, top: 700 })
+    fireEvent.pointerEnter(trigger)
+    vi.useFakeTimers()
+
+    try {
+      fireEvent.pointerDown(trigger, {
+        button: 0,
+        clientX: 964,
+        clientY: 720,
+        pointerId: 1,
+      })
+
+      expect(trigger.hasAttribute("data-dragging")).toBe(false)
+
+      act(() => {
+        vi.advanceTimersByTime(400)
+      })
+      expect(trigger.hasAttribute("data-dragging")).toBe(true)
+
+      fireEvent.pointerUp(trigger, {
+        clientX: 964,
+        clientY: 720,
+        pointerId: 1,
+      })
+      fireEvent.click(trigger)
+
+      expect(trigger.hasAttribute("data-dragging")).toBe(false)
+      expect(screen.queryByRole("dialog")).toBeNull()
+    } finally {
+      vi.useRealTimers()
+    }
+  })
+
+  it("keeps a freely dragged trigger in place without opening the panel", async () => {
+    renderDevtool()
+    const trigger = await screen.findByRole("button", {
+      name: "Open I18n Devtool",
+    })
+
+    prepareTriggerForDragging(trigger, { left: 900, top: 700 })
+    fireEvent.pointerDown(trigger, {
+      button: 0,
+      clientX: 920,
+      clientY: 720,
+      pointerId: 1,
+    })
+    fireEvent.pointerMove(trigger, {
+      clientX: 420,
+      clientY: 270,
+      pointerId: 1,
+    })
+    expect(trigger.hasAttribute("data-dragging")).toBe(true)
+    fireEvent.pointerUp(trigger, {
+      clientX: 420,
+      clientY: 270,
+      pointerId: 1,
+    })
+    fireEvent.click(trigger)
+
+    expect(trigger.hasAttribute("data-dock")).toBe(false)
+    expect(trigger.style.left).toBe("400px")
+    expect(trigger.style.top).toBe("250px")
+    expect(screen.queryByRole("dialog")).toBeNull()
+  })
+
+  it("docks and collapses after being dragged to a viewport edge", async () => {
+    renderDevtool()
+    const trigger = await screen.findByRole("button", {
+      name: "Open I18n Devtool",
+    })
+
+    prepareTriggerForDragging(trigger, { left: 400, top: 250 })
+    fireEvent.focus(trigger)
+    fireEvent.pointerEnter(trigger)
+    fireEvent.pointerDown(trigger, {
+      button: 0,
+      clientX: 420,
+      clientY: 270,
+      pointerId: 1,
+    })
+    fireEvent.pointerMove(trigger, {
+      clientX: 20,
+      clientY: 270,
+      pointerId: 1,
+    })
+    fireEvent.pointerUp(trigger, {
+      clientX: 20,
+      clientY: 270,
+      pointerId: 1,
+    })
+    fireEvent.pointerLeave(trigger)
+
+    expect(trigger.getAttribute("data-dock")).toBe("left")
+    expect(trigger.hasAttribute("data-collapsed")).toBe(true)
+    expect(trigger.style.left).toBe("0px")
+  })
+
+  it("restores its persisted floating position after remounting", async () => {
+    const firstRender = renderDevtool()
+    const trigger = await screen.findByRole("button", {
+      name: "Open I18n Devtool",
+    })
+
+    prepareTriggerForDragging(trigger, { left: 900, top: 700 })
+    fireEvent.pointerDown(trigger, {
+      button: 0,
+      clientX: 920,
+      clientY: 720,
+      pointerId: 1,
+    })
+    fireEvent.pointerMove(trigger, {
+      clientX: 420,
+      clientY: 270,
+      pointerId: 1,
+    })
+    fireEvent.pointerUp(trigger, {
+      clientX: 420,
+      clientY: 270,
+      pointerId: 1,
+    })
+
+    await waitFor(() => {
+      expect(
+        window.localStorage.getItem("i18n-devtool:floating-trigger-placement")
+      ).toContain('"x":400')
+    })
+
+    firstRender.unmount()
+    renderDevtool()
+
+    const restoredTrigger = await screen.findByRole("button", {
+      name: "Open I18n Devtool",
+    })
+
+    expect(restoredTrigger.hasAttribute("data-dock")).toBe(false)
+    expect(restoredTrigger.style.left).toBe("400px")
+    expect(restoredTrigger.style.top).toBe("250px")
+  })
+
+  it("switches between automatic, light, and dark themes", async () => {
+    mockDevtoolApi()
+    renderDevtool({ defaultOpen: true })
+    const devtool = await screen
+      .findByRole("dialog")
+      .then((dialog) =>
+        dialog.closest<HTMLElement>('[data-slot="i18n-devtool"]')
+      )
+    fireEvent.click(
+      screen.getByRole("button", {
+        name: "Open I18n Devtool settings",
+      })
+    )
+    const autoButton = screen.getByRole("button", { name: "Auto theme" })
+    const lightButton = screen.getByRole("button", { name: "Light theme" })
+    const darkButton = screen.getByRole("button", { name: "Dark theme" })
+
+    expect(autoButton.getAttribute("aria-pressed")).toBe("true")
+    expect(devtool?.getAttribute("data-theme")).toBe("auto")
+
+    fireEvent.click(darkButton)
+    expect(darkButton.getAttribute("aria-pressed")).toBe("true")
+    expect(devtool?.hasAttribute("data-dark")).toBe(true)
+
+    fireEvent.click(lightButton)
+    expect(lightButton.getAttribute("aria-pressed")).toBe("true")
+    expect(devtool?.hasAttribute("data-dark")).toBe(false)
+  })
+
+  it("positions the panel from settings and restores the preference", async () => {
+    mockDevtoolApi()
+    const firstRender = renderDevtool({ defaultOpen: true })
+    const dialog = await screen.findByRole("dialog")
+
+    expect(dialog.getAttribute("data-placement")).toBe("right")
+    fireEvent.click(
+      screen.getByRole("button", {
+        name: "Open I18n Devtool settings",
+      })
+    )
+    fireEvent.click(screen.getByRole("button", { name: "Center" }))
+    expect(dialog.getAttribute("data-placement")).toBe("center")
+
+    fireEvent.click(screen.getByRole("button", { name: "Left" }))
+    expect(dialog.getAttribute("data-placement")).toBe("left")
+
+    await waitFor(() => {
+      expect(window.localStorage.getItem("i18n-devtool:preferences")).toContain(
+        '"panelPlacement":"left"'
+      )
+    })
+
+    firstRender.unmount()
+    renderDevtool({ defaultOpen: true })
+
+    expect(
+      (await screen.findByRole("dialog")).getAttribute("data-placement")
+    ).toBe("left")
+  })
+
+  it("switches and restores the Devtool interface locale from settings", async () => {
+    mockDevtoolApi()
+    const firstRender = renderDevtool({ defaultOpen: true })
+
+    fireEvent.click(
+      await screen.findByRole("button", {
+        name: "Open I18n Devtool settings",
+      })
+    )
+
+    fireEvent.change(
+      screen.getByRole("combobox", {
+        name: "I18n Devtool interface language",
+      }),
+      { target: { value: "zh-Hans" } }
+    )
+
+    const dialog = screen.getByRole("dialog")
+    const devtool = dialog.closest<HTMLElement>('[data-slot="i18n-devtool"]')
+
+    expect(devtool?.getAttribute("lang")).toBe("zh-Hans")
+    expect(screen.getByText("界面语言")).toBeTruthy()
+    expect(screen.getByPlaceholderText("搜索消息")).toBeTruthy()
+
+    await waitFor(() => {
+      expect(window.localStorage.getItem("i18n-devtool:preferences")).toContain(
+        '"interfaceLocale":"zh-Hans"'
+      )
+    })
+
+    firstRender.unmount()
+    renderDevtool({ defaultOpen: true })
+
+    const restoredDialog = await screen.findByRole("dialog")
+    expect(
+      restoredDialog
+        .closest<HTMLElement>('[data-slot="i18n-devtool"]')
+        ?.getAttribute("lang")
+    ).toBe("zh-Hans")
+  })
+
+  it("hides panel placement settings in standalone mode", async () => {
+    mockDevtoolApi()
+    renderDevtool({
+      defaultOpen: true,
+      locale: "zh-Hans",
+      mode: "standalone",
+    })
+
+    fireEvent.click(
+      await screen.findByRole("button", {
+        name: "打开 I18n 开发工具设置",
+      })
+    )
+
+    expect(screen.getByText("主题")).toBeTruthy()
+    expect(screen.getByText("界面语言")).toBeTruthy()
+    expect(screen.queryByText("面板位置")).toBeNull()
+    expect(
+      screen.queryByRole("group", {
+        name: "I18n 开发工具面板位置",
+      })
+    ).toBeNull()
+  })
+
+  it("closes a floating panel when pressing outside it", async () => {
+    mockDevtoolApi()
+    renderDevtool({ defaultOpen: true })
+
+    const dialog = await screen.findByRole("dialog")
+
+    fireEvent.pointerDown(dialog)
+    expect(screen.getByRole("dialog")).toBeTruthy()
+
+    fireEvent.pointerDown(document.body)
+
+    await waitFor(() => {
+      expect(screen.queryByRole("dialog")).toBeNull()
+    })
+    expect(
+      screen.getByRole("button", { name: "Open I18n Devtool" })
+    ).toBeTruthy()
+  })
+
+  it("keeps a standalone panel open when pressing outside it", async () => {
+    mockDevtoolApi()
+    renderDevtool({ mode: "standalone" })
+
+    const dialog = await screen.findByRole("dialog")
+    fireEvent.pointerDown(document.body)
+
+    expect(screen.getByRole("dialog")).toBe(dialog)
+  })
+
+  it("collapses, expands, and closes command output", async () => {
+    mockDevtoolApi("\u001b[32mCatalog extraction complete\u001b[39m")
+    renderDevtool({ defaultOpen: true })
+
+    fireEvent.click(await screen.findByRole("button", { name: "Extract" }))
+    const output = await screen.findByText("Catalog extraction complete")
+
+    expect(output.tagName).toBe("SPAN")
+    expect(output.getAttribute("style")).toContain("color:")
+    expect(output.textContent).not.toContain("\u001b")
+
+    fireEvent.click(
+      screen.getByRole("button", { name: "Collapse command output" })
+    )
+    expect(screen.queryByText("Catalog extraction complete")).toBeNull()
+
+    fireEvent.click(
+      screen.getByRole("button", { name: "Expand command output" })
+    )
+    expect(screen.getByText("Catalog extraction complete")).toBeTruthy()
+
+    fireEvent.click(
+      screen.getByRole("button", { name: "Close command output" })
+    )
+    expect(screen.queryByText("Command output")).toBeNull()
+  })
+
+  it("refreshes compiled messages without remounting the message panel", async () => {
+    mockDevtoolApi("Compilation complete")
+    renderDevtool({ defaultOpen: true })
+
+    fireEvent.change(await screen.findByPlaceholderText("Search messages"), {
+      target: { value: "navigation" },
+    })
+    fireEvent.click(screen.getByRole("button", { name: "Compile" }))
+
+    await screen.findByText("Compilation complete")
+    expect(screen.getByPlaceholderText("Search messages")).toHaveProperty(
+      "value",
+      "navigation"
+    )
+  })
 })

packages/i18n/src/devtool/i18n-devtool.tsx

@@ -1,9 +1,28 @@
 import * as React from "react"
+import { Maximize2Icon, Minimize2Icon, XIcon } from "lucide-react"
 import { createPortal } from "react-dom"
 
 import { useLocale, useLocales } from "../runtime"
-import { runDevtoolAction, type DevtoolAction } from "./api-client"
+import {
+  requestDevtoolProject,
+  runDevtoolAction,
+  type DevtoolAction,
+} from "./api-client"
 import { createHttpMessageRepository } from "./http-message-repository"
+import { DevtoolActionOutput } from "./devtool-action-output"
+import { DevtoolLocaleSelect } from "./devtool-locale-select"
+import {
+  useDevtoolPreferences,
+  type I18nDevtoolPanelPlacement,
+  type I18nDevtoolTheme,
+} from "./devtool-preferences"
+import {
+  DevtoolLocalizationProvider,
+  getI18nDevtoolMessages,
+  resolveI18nDevtoolLocale,
+} from "./devtool-localization"
+import { DevtoolSettings } from "./devtool-settings"
+import { I18nDevtoolTrigger } from "./i18n-devtool-trigger"
 import { MessagePanel } from "./message-panel"
 import { MessageRepositoryProvider } from "./message-repository"
 import "./i18n-devtool.css"
@@ -13,7 +32,20 @@ export interface I18nDevtoolProps {
   className?: string
   dark?: string
   defaultOpen?: boolean
+  defaultPanelPlacement?: I18nDevtoolPanelPlacement
+  defaultTheme?: I18nDevtoolTheme
+  /**
+   * Initial locale used by the Devtool controls. The user can override it in
+   * settings. When omitted, the first supported locale from
+   * `navigator.languages` is used, with English as the fallback.
+   */
+  locale?: string
   mode?: "floating" | "standalone"
+  /**
+   * Source locale used by the project. It is excluded from the editable target
+   * locale list. When omitted, the Devtool reads it from its project endpoint.
+   */
+  sourceLocale?: string
 }
 
 function matchesDarkSelector(element: Element, dark: string) {
@@ -73,38 +105,98 @@ export function I18nDevtool({
   className,
   dark,
   defaultOpen = false,
+  defaultPanelPlacement = "right",
+  defaultTheme = "auto",
+  locale: devtoolLocale,
   mode = "floating",
+  sourceLocale,
 }: I18nDevtoolProps) {
   const activeLocale = useLocale()
   const locales = useLocales()
+  const defaultInterfaceLocale = resolveI18nDevtoolLocale(devtoolLocale)
+  const [projectSourceLocale, setProjectSourceLocale] =
+    React.useState(sourceLocale)
   const [portalTarget, setPortalTarget] = React.useState<HTMLElement | null>(
     null
   )
+  const panelRef = React.useRef<HTMLElement>(null)
   const [isOpen, setOpen] = React.useState(defaultOpen || mode === "standalone")
-  const [locale, setLocale] = React.useState(activeLocale)
-  const [reloadKey, setReloadKey] = React.useState(0)
+  const [messageLocale, setMessageLocale] = React.useState(
+    () => activeLocale ?? locales[0]?.locale ?? ""
+  )
+  const [messageRevision, setMessageRevision] = React.useState(0)
   const [actionState, setActionState] = React.useState<DevtoolAction | "idle">(
     "idle"
   )
   const [actionOutput, setActionOutput] = React.useState("")
+  const [isActionOutputExpanded, setActionOutputExpanded] = React.useState(true)
+  const [isFullscreen, setFullscreen] = React.useState(false)
   const [error, setError] = React.useState("")
-  const isDark = useDarkMode(dark)
+  const {
+    interfaceLocale: resolvedDevtoolLocale,
+    panelPlacement,
+    setInterfaceLocale,
+    setPanelPlacement,
+    setTheme,
+    theme,
+  } = useDevtoolPreferences({
+    defaultInterfaceLocale,
+    defaultPanelPlacement,
+    defaultTheme,
+  })
+  const messages = getI18nDevtoolMessages(resolvedDevtoolLocale)
+  const prefersDark = useDarkMode(dark)
+  const isDark = theme === "dark" || (theme === "auto" && prefersDark)
   const repository = React.useMemo(
     () => createHttpMessageRepository(apiBaseUrl),
     [apiBaseUrl]
   )
+  const sourceLocaleLabel = projectSourceLocale
+    ? (locales.find((definition) => definition.locale === projectSourceLocale)
+        ?.label ?? projectSourceLocale)
+    : undefined
 
   React.useEffect(() => {
     setPortalTarget(document.body)
   }, [])
 
   React.useEffect(() => {
-    if (locales.some((definition) => definition.locale === locale)) {
+    if (sourceLocale) {
+      setProjectSourceLocale(sourceLocale)
       return
     }
 
-    setLocale(activeLocale)
-  }, [activeLocale, locale, locales])
+    let cancelled = false
+
+    setProjectSourceLocale(undefined)
+    void requestDevtoolProject(apiBaseUrl).then(
+      (project) => {
+        if (!cancelled && typeof project.sourceLocale === "string") {
+          setProjectSourceLocale(project.sourceLocale)
+        }
+      },
+      () => {
+        // The rest of the Devtool remains usable when project metadata is not
+        // exposed by a custom backend.
+      }
+    )
+
+    return () => {
+      cancelled = true
+    }
+  }, [apiBaseUrl, sourceLocale])
+
+  React.useEffect(() => {
+    if (locales.some((definition) => definition.locale === messageLocale)) {
+      return
+    }
+
+    const nextLocale =
+      locales.find((definition) => definition.locale === activeLocale) ??
+      locales[0]
+
+    setMessageLocale(nextLocale?.locale ?? "")
+  }, [activeLocale, locales, messageLocale])
 
   React.useEffect(() => {
     if (!isOpen || mode === "standalone") {
@@ -113,25 +205,67 @@ export function I18nDevtool({
 
     const handleKeyDown = (event: KeyboardEvent) => {
       if (event.key === "Escape") {
-        setOpen(false)
+        if (isFullscreen) {
+          setFullscreen(false)
+        } else {
+          setOpen(false)
+        }
       }
     }
 
     window.addEventListener("keydown", handleKeyDown)
     return () => window.removeEventListener("keydown", handleKeyDown)
+  }, [isFullscreen, isOpen, mode])
+
+  React.useEffect(() => {
+    if (!isOpen || mode === "standalone") {
+      return
+    }
+
+    const handlePointerDown = (event: PointerEvent) => {
+      if (
+        event.target instanceof Node &&
+        !panelRef.current?.contains(event.target)
+      ) {
+        setFullscreen(false)
+        setOpen(false)
+      }
+    }
+
+    document.addEventListener("pointerdown", handlePointerDown, true)
+
+    return () => {
+      document.removeEventListener("pointerdown", handlePointerDown, true)
+    }
   }, [isOpen, mode])
 
   const runAction = async (action: DevtoolAction) => {
     setActionState(action)
-    setActionOutput("")
+    setActionOutput(
+      action === "extract"
+        ? messages.actions.extracting
+        : messages.actions.compiling
+    )
+    setActionOutputExpanded(true)
     setError("")
 
     try {
       const result = await runDevtoolAction(action, apiBaseUrl)
-      setActionOutput(result.output.trim() || `${action} completed.`)
-      setReloadKey((current) => current + 1)
+      setActionOutput(
+        result.output.trim() ||
+          (action === "extract"
+            ? messages.actions.extractCompleted
+            : messages.actions.compileCompleted)
+      )
+      setMessageRevision((current) => current + 1)
     } catch (reason) {
-      setError(reason instanceof Error ? reason.message : `${action} failed.`)
+      setError(
+        reason instanceof Error
+          ? reason.message
+          : action === "extract"
+            ? messages.actions.extractFailed
+            : messages.actions.compileFailed
+      )
     } finally {
       setActionState("idle")
     }
@@ -142,99 +276,165 @@ export function I18nDevtool({
   }
 
   const rootClassName = ["i18n-devtool", className].filter(Boolean).join(" ")
+  const closeActionOutput = () => {
+    setActionOutput("")
+    setError("")
+  }
+  const closePanel = () => {
+    setFullscreen(false)
+    setOpen(false)
+  }
 
   return createPortal(
-    <div
-      className={rootClassName}
-      data-dark={isDark || undefined}
-      data-mode={mode}
-      data-slot="i18n-devtool"
-    >
-      {mode === "floating" && !isOpen && (
-        <button
-          type="button"
-          aria-controls="i18n-devtool-panel"
-          aria-expanded="false"
-          className="i18n-devtool__trigger"
-          onClick={() => setOpen(true)}
-        >
-          <span aria-hidden="true">文</span>
-          <span>i18n</span>
-        </button>
-      )}
+    <DevtoolLocalizationProvider locale={resolvedDevtoolLocale}>
+      <div
+        className={rootClassName}
+        data-dark={isDark || undefined}
+        data-mode={mode}
+        data-slot="i18n-devtool"
+        data-theme={theme}
+        lang={resolvedDevtoolLocale}
+      >
+        {mode === "floating" && (
+          <I18nDevtoolTrigger open={isOpen} onOpen={() => setOpen(true)} />
+        )}
 
-      {isOpen && (
-        <section
-          id="i18n-devtool-panel"
-          aria-label="I18n Devtool"
-          className="i18n-devtool__panel"
-          role="dialog"
+        <React.Activity
+          mode={isOpen ? "visible" : "hidden"}
+          name="I18n Devtool panel"
         >
-          <header className="i18n-devtool__header">
-            <div>
-              <p className="i18n-devtool__eyebrow">Lingui</p>
-              <h2>I18n Devtool</h2>
-            </div>
+          <section
+            id="i18n-devtool-panel"
+            aria-label="I18n Devtool"
+            className="i18n-devtool__panel"
+            data-fullscreen={mode === "standalone" || isFullscreen || undefined}
+            data-placement={panelPlacement}
+            ref={panelRef}
+            role="dialog"
+          >
+            <header className="i18n-devtool__header">
+              <div>
+                <p className="i18n-devtool__eyebrow">Lingui</p>
+                <h2>I18n Devtool</h2>
+              </div>
 
-            <div className="i18n-devtool__controls">
-              <label>
-                <span>Locale</span>
-                <select
-                  aria-label="Devtool locale"
-                  value={locale}
-                  onChange={(event) => setLocale(event.target.value)}
-                >
-                  {locales.map((definition) => (
-                    <option key={definition.locale} value={definition.locale}>
-                      {definition.label ?? definition.locale}
-                    </option>
-                  ))}
-                </select>
-              </label>
-              <button
-                type="button"
-                disabled={actionState !== "idle"}
-                onClick={() => void runAction("extract")}
-              >
-                {actionState === "extract" ? "Extracting…" : "Extract"}
-              </button>
-              <button
-                type="button"
-                disabled={actionState !== "idle"}
-                onClick={() => void runAction("compile")}
-              >
-                {actionState === "compile" ? "Compiling…" : "Compile"}
-              </button>
-              {mode === "floating" && (
-                <button
-                  type="button"
-                  aria-label="Close I18n Devtool"
-                  className="i18n-devtool__close"
-                  onClick={() => setOpen(false)}
-                >
-                  ×
-                </button>
+              <div className="i18n-devtool__controls">
+                <DevtoolLocaleSelect
+                  locale={messageLocale}
+                  locales={locales}
+                  sourceLocaleLabel={sourceLocaleLabel}
+                  onLocaleChange={setMessageLocale}
+                />
+                <fieldset className="i18n-devtool__action-field">
+                  <legend>{messages.actions.label}</legend>
+                  <div className="i18n-devtool__action-controls">
+                    <button
+                      type="button"
+                      className="i18n-devtool__action-button"
+                      disabled={actionState !== "idle"}
+                      onClick={() => void runAction("extract")}
+                    >
+                      <span>
+                        {actionState === "extract"
+                          ? messages.actions.extracting
+                          : messages.actions.extract}
+                      </span>
+                      <span
+                        aria-hidden="true"
+                        className="i18n-devtool__action-label-measure"
+                      >
+                        {messages.actions.extracting}
+                      </span>
+                    </button>
+                    <button
+                      type="button"
+                      className="i18n-devtool__action-button"
+                      disabled={actionState !== "idle"}
+                      onClick={() => void runAction("compile")}
+                    >
+                      <span>
+                        {actionState === "compile"
+                          ? messages.actions.compiling
+                          : messages.actions.compile}
+                      </span>
+                      <span
+                        aria-hidden="true"
+                        className="i18n-devtool__action-label-measure"
+                      >
+                        {messages.actions.compiling}
+                      </span>
+                    </button>
+                    <DevtoolSettings
+                      interfaceLocale={resolvedDevtoolLocale}
+                      panelPlacement={panelPlacement}
+                      showPanelPlacement={mode === "floating"}
+                      theme={theme}
+                      onInterfaceLocaleChange={setInterfaceLocale}
+                      onPanelPlacementChange={setPanelPlacement}
+                      onThemeChange={setTheme}
+                    />
+                    {mode === "floating" && (
+                      <button
+                        type="button"
+                        aria-label={
+                          isFullscreen
+                            ? messages.actions.exitFullscreen
+                            : messages.actions.enterFullscreen
+                        }
+                        aria-pressed={isFullscreen}
+                        className="i18n-devtool__fullscreen"
+                        onClick={() => setFullscreen((current) => !current)}
+                      >
+                        {isFullscreen ? (
+                          <Minimize2Icon aria-hidden="true" />
+                        ) : (
+                          <Maximize2Icon aria-hidden="true" />
+                        )}
+                      </button>
+                    )}
+                    {mode === "floating" && (
+                      <button
+                        type="button"
+                        aria-label={messages.trigger.close}
+                        className="i18n-devtool__close"
+                        onClick={closePanel}
+                      >
+                        <XIcon aria-hidden="true" />
+                      </button>
+                    )}
+                  </div>
+                </fieldset>
+              </div>
+            </header>
+
+            {(error || actionOutput) && (
+              <DevtoolActionOutput
+                error={Boolean(error)}
+                expanded={isActionOutputExpanded}
+                output={error || actionOutput}
+                onClose={closeActionOutput}
+                onExpandedChange={setActionOutputExpanded}
+              />
+            )}
+
+            <main className="i18n-devtool__main">
+              {messageLocale ? (
+                <MessageRepositoryProvider repository={repository}>
+                  <MessagePanel
+                    locale={messageLocale}
+                    refreshToken={messageRevision}
+                  />
+                </MessageRepositoryProvider>
+              ) : (
+                <div className="i18n-devtool__empty">
+                  {messages.locale.noTargetLocales}
+                </div>
               )}
-            </div>
-          </header>
-
-          {(error || actionOutput) && (
-            <pre
-              className="i18n-devtool__output"
-              data-error={error ? "" : undefined}
-            >
-              {error || actionOutput}
-            </pre>
-          )}
-
-          <main className="i18n-devtool__main">
-            <MessageRepositoryProvider repository={repository}>
-              <MessagePanel key={`${locale}:${reloadKey}`} locale={locale} />
-            </MessageRepositoryProvider>
-          </main>
-        </section>
-      )}
-    </div>,
+            </main>
+          </section>
+        </React.Activity>
+      </div>
+    </DevtoolLocalizationProvider>,
     portalTarget
   )
 }

packages/i18n/src/devtool/index.ts

@@ -1,5 +1,18 @@
 export { createHttpMessageRepository } from "./http-message-repository"
 export { I18nDevtool, type I18nDevtoolProps } from "./i18n-devtool"
+export {
+  I18nDevtoolTrigger,
+  type I18nDevtoolTriggerProps,
+} from "./i18n-devtool-trigger"
+export {
+  I18N_DEVTOOL_LOCALES,
+  type I18nDevtoolLocale,
+  resolveI18nDevtoolLocale,
+} from "./devtool-localization"
+export type {
+  I18nDevtoolPanelPlacement,
+  I18nDevtoolTheme,
+} from "./devtool-preferences"
 export { MessagePanel, type MessagePanelProps } from "./message-panel"
 export {
   MessageRepositoryProvider,
@@ -8,6 +21,7 @@ export {
 } from "./message-repository"
 export type {
   DevtoolMessage,
+  DevtoolMessagePackage,
   MessageOrigin,
   MessageRepository,
   UpdateMessageInput,

packages/i18n/src/devtool/message-copy-menu.tsx

@@ -0,0 +1,285 @@
+import * as React from "react"
+import {
+  CheckIcon,
+  ChevronDownIcon,
+  CopyIcon,
+  TriangleAlertIcon,
+} from "lucide-react"
+
+import { useDevtoolLocalization } from "./devtool-localization"
+import type { DevtoolMessage, MessageOrigin } from "./types"
+
+function getOriginLocation(origin: MessageOrigin) {
+  return `${origin.file}${origin.line ? `:${origin.line}` : ""}`
+}
+
+export function formatMessageOrigin(origin: MessageOrigin) {
+  const normalizedFile = origin.file.replaceAll("\\", "/")
+  const packageRelativeFile =
+    normalizedFile.match(/(?:^|\/)(?:apps|packages)\/[^/]+\/(.+)$/)?.[1] ??
+    normalizedFile.replace(/^(?:\.\.\/)+/, "")
+
+  return `${packageRelativeFile}${origin.line ? `:${origin.line}` : ""}`
+}
+
+function serializeMessageDescriptor(message: DevtoolMessage) {
+  return [
+    "{",
+    `  id: ${JSON.stringify(message.id)},`,
+    `  message: ${JSON.stringify(message.source || message.id)},`,
+    "}",
+  ].join("\n")
+}
+
+async function writeClipboard(value: string) {
+  if (navigator.clipboard?.writeText) {
+    await navigator.clipboard.writeText(value)
+    return
+  }
+
+  const textarea = document.createElement("textarea")
+  textarea.value = value
+  textarea.style.position = "fixed"
+  textarea.style.opacity = "0"
+  document.body.appendChild(textarea)
+  textarea.select()
+
+  try {
+    if (!document.execCommand("copy")) {
+      throw new Error("The browser rejected the clipboard operation.")
+    }
+  } finally {
+    textarea.remove()
+  }
+}
+
+export function MessageCopyMenu({
+  message,
+  translation,
+}: {
+  message: DevtoolMessage
+  translation: string
+}) {
+  const { messages } = useDevtoolLocalization()
+  const [isOpen, setOpen] = React.useState(false)
+  const [copyState, setCopyState] = React.useState<"idle" | "copied" | "error">(
+    "idle"
+  )
+  const rootRef = React.useRef<HTMLDivElement>(null)
+  const triggerRef = React.useRef<HTMLButtonElement>(null)
+
+  React.useEffect(() => {
+    if (!isOpen) {
+      return
+    }
+
+    const handlePointerDown = (event: PointerEvent) => {
+      if (
+        event.target instanceof Node &&
+        !rootRef.current?.contains(event.target)
+      ) {
+        setOpen(false)
+      }
+    }
+    const handleKeyDown = (event: KeyboardEvent) => {
+      if (event.key === "Escape") {
+        event.preventDefault()
+        setOpen(false)
+        triggerRef.current?.focus()
+      }
+    }
+
+    document.addEventListener("pointerdown", handlePointerDown)
+    document.addEventListener("keydown", handleKeyDown)
+
+    return () => {
+      document.removeEventListener("pointerdown", handlePointerDown)
+      document.removeEventListener("keydown", handleKeyDown)
+    }
+  }, [isOpen])
+
+  React.useEffect(() => {
+    if (copyState === "idle") {
+      return
+    }
+
+    const timeout = window.setTimeout(() => setCopyState("idle"), 1600)
+    return () => window.clearTimeout(timeout)
+  }, [copyState])
+
+  const openMenu = () => {
+    setOpen(true)
+    window.setTimeout(() => {
+      rootRef.current
+        ?.querySelector<HTMLButtonElement>('[role="menuitem"]:not(:disabled)')
+        ?.focus()
+    })
+  }
+
+  const closeMenu = () => {
+    setOpen(false)
+    triggerRef.current?.focus()
+  }
+
+  const copy = async (value: string) => {
+    try {
+      await writeClipboard(value)
+      setCopyState("copied")
+    } catch {
+      setCopyState("error")
+    } finally {
+      closeMenu()
+    }
+  }
+
+  const handleMenuKeyDown = (event: React.KeyboardEvent<HTMLDivElement>) => {
+    if (!["ArrowDown", "ArrowUp", "End", "Home"].includes(event.key)) {
+      return
+    }
+
+    event.preventDefault()
+    const items = Array.from(
+      event.currentTarget.querySelectorAll<HTMLButtonElement>(
+        '[role="menuitem"]:not(:disabled)'
+      )
+    )
+
+    if (items.length === 0) {
+      return
+    }
+
+    const currentIndex = items.indexOf(
+      document.activeElement as HTMLButtonElement
+    )
+    const nextIndex =
+      event.key === "Home"
+        ? 0
+        : event.key === "End"
+          ? items.length - 1
+          : event.key === "ArrowUp"
+            ? (currentIndex - 1 + items.length) % items.length
+            : (currentIndex + 1) % items.length
+
+    items[nextIndex]?.focus()
+  }
+
+  return (
+    <div className="i18n-message-copy" ref={rootRef}>
+      <button
+        type="button"
+        aria-expanded={isOpen}
+        aria-haspopup="menu"
+        aria-label={
+          copyState === "copied"
+            ? messages.messagePanel.copied
+            : copyState === "error"
+              ? messages.messagePanel.copyFailed
+              : messages.messagePanel.chooseCopyContent
+        }
+        className="i18n-message-copy__trigger"
+        data-state={copyState}
+        ref={triggerRef}
+        title={messages.messagePanel.chooseCopyContent}
+        onClick={() => {
+          if (isOpen) {
+            closeMenu()
+          } else {
+            openMenu()
+          }
+        }}
+      >
+        {copyState === "copied" ? (
+          <CheckIcon aria-hidden="true" />
+        ) : copyState === "error" ? (
+          <TriangleAlertIcon aria-hidden="true" />
+        ) : (
+          <CopyIcon aria-hidden="true" />
+        )}
+        <ChevronDownIcon
+          aria-hidden="true"
+          className="i18n-message-copy__chevron"
+        />
+      </button>
+
+      {isOpen && (
+        <div
+          className="i18n-message-copy__menu"
+          role="menu"
+          onKeyDown={handleMenuKeyDown}
+        >
+          <button
+            type="button"
+            role="menuitem"
+            onClick={() => void copy(message.id)}
+          >
+            {messages.messagePanel.copyMessageId}
+          </button>
+          <button
+            type="button"
+            role="menuitem"
+            onClick={() => void copy(message.source || message.id)}
+          >
+            {messages.messagePanel.copySource}
+          </button>
+          <button
+            type="button"
+            disabled={translation.length === 0}
+            role="menuitem"
+            onClick={() => void copy(translation)}
+          >
+            {messages.messagePanel.copyTranslation}
+          </button>
+
+          {message.origins.length > 0 && (
+            <>
+              <div className="i18n-message-copy__separator" role="separator" />
+              <span className="i18n-message-copy__label">
+                {messages.messagePanel.sourceLocations}
+              </span>
+              {message.origins.map((origin, index) => (
+                <button
+                  type="button"
+                  key={`${getOriginLocation(origin)}:${index}`}
+                  role="menuitem"
+                  title={getOriginLocation(origin)}
+                  onClick={() => void copy(getOriginLocation(origin))}
+                >
+                  <span>{messages.messagePanel.copySourceLocation}</span>
+                  <code>{formatMessageOrigin(origin)}</code>
+                </button>
+              ))}
+            </>
+          )}
+
+          <div className="i18n-message-copy__separator" role="separator" />
+          <button
+            type="button"
+            role="menuitem"
+            onClick={() => void copy(serializeMessageDescriptor(message))}
+          >
+            {messages.messagePanel.copyDescriptor}
+          </button>
+        </div>
+      )}
+
+      {copyState !== "idle" && !isOpen && (
+        <span
+          aria-atomic="true"
+          aria-live="polite"
+          className="i18n-message-copy__feedback"
+          data-state={copyState}
+          role="status"
+        >
+          {copyState === "copied" ? (
+            <CheckIcon aria-hidden="true" />
+          ) : (
+            <TriangleAlertIcon aria-hidden="true" />
+          )}
+          {copyState === "copied"
+            ? messages.messagePanel.copied
+            : messages.messagePanel.copyFailed}
+        </span>
+      )}
+    </div>
+  )
+}

packages/i18n/src/devtool/message-navigation.tsx

@@ -0,0 +1,356 @@
+import * as React from "react"
+import { ChevronDownIcon, FolderIcon, PackageIcon } from "lucide-react"
+
+import { useDevtoolLocalization } from "./devtool-localization"
+import type { DevtoolMessage, DevtoolMessagePackage } from "./types"
+
+export interface MessagePackageGroup {
+  kind: DevtoolMessagePackage["kind"]
+  messages: readonly DevtoolMessage[]
+  missingCount: number
+  packageName: string
+}
+
+interface MessageNamespaceNode {
+  children: Map<string, MessageNamespaceNode>
+  descendantCount: number
+  messages: DevtoolMessage[]
+  path: string
+  segment: string
+}
+
+interface MessagePackageNode extends MessagePackageGroup {
+  messagesAtRoot: readonly DevtoolMessage[]
+  namespaces: Map<string, MessageNamespaceNode>
+  selectionKey: string
+}
+
+export interface MessageNavigationSelection {
+  key: string
+  label: string
+  messages: readonly DevtoolMessage[]
+  packageName: string
+}
+
+export interface MessageNavigationModel {
+  packages: readonly MessagePackageNode[]
+  selections: ReadonlyMap<string, MessageNavigationSelection>
+}
+
+function getPackageSelectionKey(packageName: string) {
+  return `package:${packageName}`
+}
+
+function getNamespaceSelectionKey(packageName: string, path: string) {
+  return `package:${packageName}:namespace:${path}`
+}
+
+export function createMessageNavigationModel(
+  groups: readonly MessagePackageGroup[]
+): MessageNavigationModel {
+  const selections = new Map<string, MessageNavigationSelection>()
+  const packages = groups.map<MessagePackageNode>((group) => {
+    const messagesAtRoot: DevtoolMessage[] = []
+    const namespaces = new Map<string, MessageNamespaceNode>()
+
+    for (const message of group.messages) {
+      const segments = message.id.split(".").filter(Boolean)
+
+      if (segments.length <= 1) {
+        messagesAtRoot.push(message)
+        continue
+      }
+
+      const namespaceSegments = segments.slice(0, -1)
+      const namespacePath = namespaceSegments.join(".")
+      let children = namespaces
+      let path = ""
+
+      for (const segment of namespaceSegments) {
+        path = path ? `${path}.${segment}` : segment
+        let node = children.get(segment)
+
+        if (!node) {
+          node = {
+            children: new Map(),
+            descendantCount: 0,
+            messages: [],
+            path,
+            segment,
+          }
+          children.set(segment, node)
+        }
+
+        node.descendantCount += 1
+        children = node.children
+
+        if (path === namespacePath) {
+          node.messages.push(message)
+        }
+      }
+    }
+
+    const selectionKey = getPackageSelectionKey(group.packageName)
+    selections.set(selectionKey, {
+      key: selectionKey,
+      label: group.packageName,
+      messages: messagesAtRoot,
+      packageName: group.packageName,
+    })
+
+    const addNamespaceSelections = (
+      nodes: Map<string, MessageNamespaceNode>
+    ) => {
+      for (const node of nodes.values()) {
+        const key = getNamespaceSelectionKey(group.packageName, node.path)
+        selections.set(key, {
+          key,
+          label: node.path,
+          messages: node.messages,
+          packageName: group.packageName,
+        })
+        addNamespaceSelections(node.children)
+      }
+    }
+
+    addNamespaceSelections(namespaces)
+
+    return {
+      ...group,
+      messagesAtRoot,
+      namespaces,
+      selectionKey,
+    }
+  })
+
+  return { packages, selections }
+}
+
+function NamespaceBranch({
+  nodes,
+  onSelect,
+  packageName,
+  selectedKey,
+}: {
+  nodes: Map<string, MessageNamespaceNode>
+  onSelect: (key: string) => void
+  packageName: string
+  selectedKey?: string
+}) {
+  return (
+    <ul className="i18n-message-tree__branch">
+      {Array.from(nodes.values())
+        .sort((left, right) => left.segment.localeCompare(right.segment))
+        .map((node) => (
+          <NamespaceNode
+            key={node.path}
+            node={node}
+            packageName={packageName}
+            selectedKey={selectedKey}
+            onSelect={onSelect}
+          />
+        ))}
+    </ul>
+  )
+}
+
+function NamespaceNode({
+  node,
+  onSelect,
+  packageName,
+  selectedKey,
+}: {
+  node: MessageNamespaceNode
+  onSelect: (key: string) => void
+  packageName: string
+  selectedKey?: string
+}) {
+  const [isExpanded, setExpanded] = React.useState(true)
+  const { messages } = useDevtoolLocalization()
+  const selectionKey = getNamespaceSelectionKey(packageName, node.path)
+  const hasChildren = node.children.size > 0
+  const hasMessages = node.messages.length > 0
+  const isSelected = selectionKey === selectedKey
+
+  return (
+    <li>
+      <div
+        className="i18n-message-tree__namespace-row"
+        data-selected={isSelected || undefined}
+      >
+        <button
+          type="button"
+          aria-expanded={hasChildren ? isExpanded : undefined}
+          aria-label={
+            hasChildren
+              ? isExpanded
+                ? messages.messagePanel.collapseNamespace(node.path)
+                : messages.messagePanel.expandNamespace(node.path)
+              : undefined
+          }
+          className="i18n-message-tree__disclosure"
+          disabled={!hasChildren}
+          onClick={() => setExpanded((current) => !current)}
+        >
+          {hasChildren && (
+            <ChevronDownIcon
+              aria-hidden="true"
+              className="i18n-message-tree__chevron"
+            />
+          )}
+        </button>
+        <button
+          type="button"
+          aria-current={isSelected ? "true" : undefined}
+          className="i18n-message-tree__namespace"
+          data-selectable={hasMessages || undefined}
+          title={node.path}
+          onClick={() => {
+            if (hasMessages) {
+              onSelect(selectionKey)
+            } else if (hasChildren) {
+              setExpanded((current) => !current)
+            }
+          }}
+        >
+          <FolderIcon aria-hidden="true" />
+          <span>{node.segment}</span>
+          <small>{node.descendantCount}</small>
+        </button>
+      </div>
+      {hasChildren && (
+        <div hidden={!isExpanded}>
+          <NamespaceBranch
+            nodes={node.children}
+            packageName={packageName}
+            selectedKey={selectedKey}
+            onSelect={onSelect}
+          />
+        </div>
+      )}
+    </li>
+  )
+}
+
+function PackageTree({
+  node,
+  onSelect,
+  selectedKey,
+}: {
+  node: MessagePackageNode
+  onSelect: (key: string) => void
+  selectedKey?: string
+}) {
+  const { messages } = useDevtoolLocalization()
+  const [isExpanded, setExpanded] = React.useState(true)
+  const contentId = React.useId()
+  const hasNamespaces = node.namespaces.size > 0
+  const hasContent = node.messages.length === 0 || hasNamespaces
+  const isSelected = node.selectionKey === selectedKey
+
+  return (
+    <section
+      aria-label={node.packageName}
+      className="i18n-message-package"
+      data-collapsed={!isExpanded || undefined}
+      data-kind={node.kind}
+      data-selected={isSelected || undefined}
+    >
+      <header className="i18n-message-package__header">
+        <button
+          type="button"
+          aria-current={isSelected ? "true" : undefined}
+          className="i18n-message-package__identity"
+          onClick={() => onSelect(node.selectionKey)}
+        >
+          <span aria-hidden="true" className="i18n-message-package__icon">
+            <PackageIcon />
+          </span>
+          <code>{node.packageName}</code>
+        </button>
+        <span className="i18n-message-package__controls">
+          <span className="i18n-message-package__counts">
+            <span>
+              {messages.messagePanel.messageCount(node.messages.length)}
+            </span>
+            {node.missingCount > 0 && (
+              <span data-missing>
+                {messages.messagePanel.missingCount(node.missingCount)}
+              </span>
+            )}
+          </span>
+          <button
+            type="button"
+            aria-controls={hasContent ? contentId : undefined}
+            aria-expanded={hasContent ? isExpanded : undefined}
+            aria-label={
+              hasContent
+                ? isExpanded
+                  ? messages.messagePanel.collapsePackage(node.packageName)
+                  : messages.messagePanel.expandPackage(node.packageName)
+                : undefined
+            }
+            className="i18n-message-package__disclosure"
+            disabled={!hasContent}
+            onClick={() => setExpanded((current) => !current)}
+          >
+            {hasContent && (
+              <ChevronDownIcon
+                aria-hidden="true"
+                className="i18n-message-package__chevron"
+              />
+            )}
+          </button>
+        </span>
+      </header>
+      {hasContent && (
+        <div
+          id={contentId}
+          className="i18n-message-package__messages"
+          hidden={!isExpanded}
+        >
+          {node.messages.length === 0 ? (
+            <div className="i18n-message-package__empty">
+              {messages.messagePanel.emptyPackage}
+            </div>
+          ) : (
+            <NamespaceBranch
+              nodes={node.namespaces}
+              packageName={node.packageName}
+              selectedKey={selectedKey}
+              onSelect={onSelect}
+            />
+          )}
+        </div>
+      )}
+    </section>
+  )
+}
+
+export function MessageNavigation({
+  model,
+  onSelect,
+  selectedKey,
+}: {
+  model: MessageNavigationModel
+  onSelect: (key: string) => void
+  selectedKey?: string
+}) {
+  const { messages } = useDevtoolLocalization()
+
+  return (
+    <nav
+      aria-label={messages.messagePanel.messageNavigation}
+      className="i18n-message-navigation"
+    >
+      {model.packages.map((node) => (
+        <PackageTree
+          key={node.packageName}
+          node={node}
+          selectedKey={selectedKey}
+          onSelect={onSelect}
+        />
+      ))}
+    </nav>
+  )
+}

packages/i18n/src/devtool/message-panel.css

@@ -3,6 +3,9 @@
   --i18n-muted: color-mix(in srgb, currentColor 62%, transparent);
   --i18n-surface: color-mix(in srgb, Canvas 96%, currentColor 4%);
   color: CanvasText;
+  display: flex;
+  min-height: 0;
+  flex-direction: column;
   font-family:
     Inter,
     ui-sans-serif,
@@ -33,8 +36,21 @@
 }
 
 .i18n-message-panel__search {
+  position: relative;
+  display: grid;
   min-width: 12rem;
   flex: 1;
+  align-items: center;
+}
+
+.i18n-message-panel__search > svg {
+  position: absolute;
+  left: 0.75rem;
+  z-index: 1;
+  width: 1rem;
+  height: 1rem;
+  color: var(--i18n-muted);
+  pointer-events: none;
 }
 
 .i18n-message-panel input,
@@ -55,16 +71,24 @@
 
 .i18n-message-panel input[type="search"] {
   height: 2.5rem;
-  padding: 0 0.75rem;
+  padding: 0 3.25rem 0 2.25rem;
 }
 
 .i18n-message-panel textarea {
-  min-height: 5.5rem;
+  min-height: 3.25rem;
+  max-height: 14rem;
+  field-sizing: content;
   resize: vertical;
-  padding: 0.75rem;
+  padding: 0.65rem 0.75rem;
   line-height: 1.5;
 }
 
+.i18n-message-panel textarea:read-only {
+  background: color-mix(in srgb, Canvas 92%, currentColor 8%);
+  color: var(--i18n-muted);
+  cursor: default;
+}
+
 .i18n-message-panel input[type="search"]:focus,
 .i18n-message-panel textarea:focus {
   border-color: AccentColor;
@@ -72,32 +96,464 @@
 }
 
 .i18n-message-panel__toggle {
+  position: relative;
   display: inline-flex;
   align-items: center;
   gap: 0.4rem;
+  padding: 0.25rem 0.4rem;
+  border-radius: 0.45rem;
+  cursor: pointer;
   white-space: nowrap;
   font-size: 0.875rem;
+  transition: background-color 120ms ease;
+}
+
+.i18n-message-panel__toggle:hover {
+  background: color-mix(in srgb, Canvas 90%, currentColor 10%);
+}
+
+.i18n-message-panel__toggle:active {
+  background: color-mix(in srgb, Canvas 84%, currentColor 16%);
+}
+
+.i18n-message-panel__toggle input {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  overflow: hidden;
+  opacity: 0;
+  pointer-events: none;
+}
+
+.i18n-message-panel__checkbox {
+  display: inline-grid;
+  width: 1.125rem;
+  height: 1.125rem;
+  flex: 0 0 1.125rem;
+  place-items: center;
+  border: 1px solid var(--i18n-border);
+  border-radius: 0.25rem;
+  background: Canvas;
+  color: white;
+  transition:
+    background-color 120ms ease,
+    border-color 120ms ease,
+    box-shadow 120ms ease,
+    transform 80ms ease;
+}
+
+.i18n-message-panel__checkbox svg {
+  width: 0.875rem;
+  height: 0.875rem;
+  opacity: 0;
+  stroke-width: 3;
+  transform: scale(0.6);
+  transition:
+    opacity 100ms ease,
+    transform 120ms ease;
+}
+
+.i18n-message-panel__toggle input:checked + .i18n-message-panel__checkbox {
+  border-color: AccentColor;
+  background: AccentColor;
+}
+
+.i18n-message-panel__toggle input:checked + .i18n-message-panel__checkbox svg {
+  opacity: 1;
+  transform: scale(1);
+}
+
+.i18n-message-panel__toggle
+  input:focus-visible
+  + .i18n-message-panel__checkbox {
+  box-shadow: 0 0 0 3px color-mix(in srgb, AccentColor 22%, transparent);
+}
+
+.i18n-message-panel__toggle:active .i18n-message-panel__checkbox {
+  transform: scale(0.92);
 }
 
 .i18n-message-panel__count {
+  display: inline-grid;
+  min-width: 1.5rem;
+  height: 1.5rem;
+  place-items: center;
+  padding: 0 0.35rem;
+  border-radius: 999px;
+  background: color-mix(in srgb, Canvas 86%, currentColor 14%);
   color: var(--i18n-muted);
   font-variant-numeric: tabular-nums;
-  font-size: 0.8125rem;
+  font-size: 0.75rem;
 }
 
-.i18n-message-panel__list {
+.i18n-message-panel__result-count {
+  position: absolute;
+  right: 0.55rem;
+  display: inline-grid;
+  min-width: 1.75rem;
+  height: 1.5rem;
+  place-items: center;
+  padding: 0 0.4rem;
+  border-radius: 999px;
+  background: color-mix(in srgb, Canvas 84%, currentColor 16%);
+  color: var(--i18n-muted);
+  font-variant-numeric: tabular-nums;
+  font-size: 0.75rem;
+  pointer-events: none;
+}
+
+.i18n-message-workspace {
   display: grid;
-  gap: 0.75rem;
+  min-height: 30rem;
+  flex: 1;
+  grid-template-columns: minmax(18rem, min(28rem, 44%)) minmax(0, 1fr);
+}
+
+.i18n-message-navigation,
+.i18n-message-workspace__editor {
+  min-width: 0;
+  min-height: 0;
+  overflow-x: hidden;
+  overflow-y: auto;
+  overscroll-behavior: contain;
+  scrollbar-gutter: stable;
+}
+
+.i18n-message-navigation {
+  display: flex;
+  flex-direction: column;
+  align-items: stretch;
+  gap: 0.65rem;
   padding: 0.75rem;
+  border-right: 1px solid var(--i18n-border);
+  background: color-mix(in srgb, Canvas 94%, currentColor 6%);
+}
+
+.i18n-message-workspace__editor {
+  padding: 0.75rem;
+}
+
+.i18n-message-workspace__editor-list {
+  display: grid;
+  align-content: start;
+  gap: 0.75rem;
+}
+
+.i18n-message-workspace__empty {
+  display: grid;
+  min-height: 100%;
+  place-items: center;
+  padding: 2rem;
+  color: var(--i18n-muted);
+  text-align: center;
+}
+
+.i18n-message-package {
+  display: grid;
+  min-width: 0;
+  flex: none;
+  overflow: hidden;
+  border: 1px solid var(--i18n-border);
+  border-radius: 0.75rem;
+  background: color-mix(in srgb, Canvas 91%, currentColor 9%);
+  box-shadow: 0 0.15rem 0.5rem rgb(0 0 0 / 4%);
+}
+
+.i18n-message-package__messages[hidden] {
+  display: none;
+}
+
+.i18n-message-package__messages {
+  position: relative;
+  display: grid;
+  padding: 0.45rem 0.45rem 0.55rem 0.7rem;
+  border-top: 1px solid var(--i18n-border);
+  background: color-mix(in srgb, Canvas 96%, currentColor 4%);
+}
+
+.i18n-message-package__empty {
+  display: grid;
+  min-height: 5rem;
+  place-items: center;
+  padding: 1rem;
+  border: 1px dashed color-mix(in srgb, currentColor 18%, transparent);
+  border-radius: 0.55rem;
+  background: color-mix(in srgb, Canvas 80%, transparent);
+  color: var(--i18n-muted);
+  font-size: 0.8125rem;
+  text-align: center;
+}
+
+.i18n-message-package__header {
+  display: flex;
+  min-width: 0;
+  min-height: 3rem;
+  align-items: center;
+  justify-content: space-between;
+  gap: 0.35rem;
+  padding: 0.3rem;
+  background: color-mix(in srgb, Canvas 86%, currentColor 14%);
+}
+
+.i18n-message-panel button.i18n-message-package__identity {
+  display: flex;
+  min-width: 0;
+  min-height: 2.35rem;
+  flex: 1;
+  justify-content: flex-start;
+  gap: 0.6rem;
+  padding: 0.25rem 0.35rem;
+  border: 0;
+  border-radius: 0.5rem;
+  outline: none;
+  background: transparent;
+  text-align: start;
+}
+
+.i18n-message-panel button.i18n-message-package__identity:hover:not(:disabled),
+.i18n-message-panel
+  .i18n-message-package[data-selected]
+  button.i18n-message-package__identity {
+  background: color-mix(in srgb, Canvas 80%, currentColor 20%);
+}
+
+.i18n-message-panel button.i18n-message-package__identity:focus-visible,
+.i18n-message-panel button.i18n-message-package__disclosure:focus-visible {
+  box-shadow: inset 0 0 0 2px AccentColor;
+}
+
+.i18n-message-package__identity code {
+  min-width: 0;
+  overflow: hidden;
+  font-size: 0.8125rem;
+  font-weight: 700;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.i18n-message-package__icon {
+  display: inline-grid;
+  width: 1.9rem;
+  height: 1.9rem;
+  flex: 0 0 1.9rem;
+  place-items: center;
+  border-radius: 0.5rem;
+  background: color-mix(in srgb, AccentColor 14%, Canvas);
+  color: AccentColor;
+}
+
+.i18n-message-package__icon svg {
+  width: 1rem;
+  height: 1rem;
+}
+
+.i18n-message-package__counts,
+.i18n-message-package__controls {
+  display: inline-flex;
+  min-width: 0;
+  align-items: center;
+}
+
+.i18n-message-package__counts {
+  flex: none;
+  gap: 0.35rem;
+}
+
+.i18n-message-package__controls {
+  flex: none;
+  gap: 0.25rem;
+}
+
+.i18n-message-panel button.i18n-message-package__disclosure {
+  width: 2rem;
+  min-height: 0;
+  height: 2rem;
+  flex: 0 0 2rem;
+  padding: 0;
+  border: 0;
+  border-radius: 0.45rem;
+  outline: none;
+  background: transparent;
+}
+
+.i18n-message-package__chevron {
+  width: 1rem;
+  height: 1rem;
+  flex: 0 0 1rem;
+  color: var(--i18n-muted);
+  transition: rotate 140ms ease;
+}
+
+.i18n-message-package__disclosure[aria-expanded="false"]
+  .i18n-message-package__chevron {
+  rotate: -90deg;
+}
+
+.i18n-message-package__disclosure[aria-expanded="false"]:dir(rtl)
+  .i18n-message-package__chevron {
+  rotate: 90deg;
+}
+
+.i18n-message-package__counts span {
+  padding: 0.2rem 0.45rem;
+  border-radius: 999px;
+  background: color-mix(in srgb, Canvas 84%, currentColor 16%);
+  color: var(--i18n-muted);
+  font-variant-numeric: tabular-nums;
+  font-size: 0.6875rem;
+  white-space: nowrap;
+}
+
+.i18n-message-package__counts span[data-missing] {
+  background: color-mix(in srgb, #e8a317 14%, Canvas);
+  color: color-mix(in srgb, #b36b00 86%, CanvasText);
+}
+
+.i18n-message-tree__branch {
+  position: relative;
+  display: grid;
+  gap: 0.2rem;
+  margin: 0;
+  padding: 0;
+  list-style: none;
+}
+
+.i18n-message-tree__branch .i18n-message-tree__branch {
+  margin-block: 0.15rem 0.25rem;
+  margin-inline-start: 0.85rem;
+  padding-inline-start: 0.75rem;
+  border-inline-start: 1px solid
+    color-mix(in srgb, currentColor 18%, transparent);
+}
+
+.i18n-message-tree__branch li {
+  min-width: 0;
+}
+
+.i18n-message-tree__namespace-row {
+  display: grid;
+  min-width: 0;
+  grid-template-columns: 1.65rem minmax(0, 1fr);
+  align-items: center;
+  border-radius: 0.5rem;
+  transition:
+    color 120ms ease,
+    background-color 120ms ease;
+}
+
+.i18n-message-tree__namespace-row:hover {
+  background: color-mix(in srgb, Canvas 82%, currentColor 18%);
+}
+
+.i18n-message-tree__namespace-row[data-selected] {
+  background: color-mix(in srgb, AccentColor 14%, Canvas);
+  color: color-mix(in srgb, AccentColor 82%, CanvasText);
+}
+
+.i18n-message-panel button.i18n-message-tree__disclosure {
+  width: 1.65rem;
+  min-height: 1.9rem;
+  padding: 0;
+  border: 0;
+  border-radius: 0.4rem;
+  outline: none;
+  background: transparent;
+}
+
+.i18n-message-panel button.i18n-message-tree__disclosure:focus-visible,
+.i18n-message-panel button.i18n-message-tree__namespace:focus-visible {
+  box-shadow: inset 0 0 0 2px AccentColor;
+}
+
+.i18n-message-panel button.i18n-message-tree__namespace {
+  display: flex;
+  width: 100%;
+  min-width: 0;
+  min-height: 2.15rem;
+  align-items: center;
+  justify-content: flex-start;
+  gap: 0.45rem;
+  padding: 0.35rem 0.5rem 0.35rem 0.2rem;
+  border: 0;
+  border-radius: 0.5rem;
+  outline: none;
+  background: transparent;
+  text-align: start;
+}
+
+.i18n-message-panel button.i18n-message-tree__namespace[data-selectable] {
+  font-weight: 600;
+}
+
+.i18n-message-tree__namespace > span {
+  min-width: 0;
+  flex: 1;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.i18n-message-tree__namespace small {
+  margin-inline-start: auto;
+  color: var(--i18n-muted);
+  font-variant-numeric: tabular-nums;
+  font-size: 0.6875rem;
+}
+
+.i18n-message-tree__namespace > svg {
+  width: 0.9rem;
+  height: 0.9rem;
+  flex: 0 0 0.9rem;
+  color: var(--i18n-muted);
+}
+
+.i18n-message-tree__chevron {
+  width: 0.9rem;
+  height: 0.9rem;
+  color: var(--i18n-muted);
+  transition: rotate 120ms ease;
+}
+
+.i18n-message-tree__disclosure[aria-expanded="false"]
+  .i18n-message-tree__chevron {
+  rotate: -90deg;
+}
+
+.i18n-message-tree__disclosure[aria-expanded="false"]:dir(rtl)
+  .i18n-message-tree__chevron {
+  rotate: 90deg;
 }
 
 .i18n-message {
   display: grid;
-  gap: 0.875rem;
-  padding: 1rem;
+  gap: 0.65rem;
+  overflow: visible;
+  padding: 0.875rem;
   border: 1px solid var(--i18n-border);
-  border-radius: 0.875rem;
+  border-radius: 0.75rem;
   background: var(--i18n-surface);
+  transition:
+    border-color 120ms ease,
+    box-shadow 120ms ease,
+    background-color 120ms ease;
+}
+
+.i18n-message:not(:has(.i18n-message-copy__trigger[aria-expanded="true"])) {
+  content-visibility: auto;
+  contain-intrinsic-block-size: 13rem;
+}
+
+.i18n-message:hover {
+  border-color: color-mix(in srgb, currentColor 22%, transparent);
+}
+
+.i18n-message:focus-within {
+  border-color: color-mix(in srgb, AccentColor 55%, var(--i18n-border));
+  box-shadow: 0 0 0 3px color-mix(in srgb, AccentColor 10%, transparent);
+}
+
+.i18n-message[data-dirty] {
+  border-color: color-mix(in srgb, AccentColor 42%, var(--i18n-border));
 }
 
 .i18n-message[data-missing] {
@@ -108,6 +564,210 @@
   opacity: 0.65;
 }
 
+.i18n-message__source-toolbar {
+  display: flex;
+  min-width: 0;
+  align-items: center;
+  justify-content: space-between;
+  gap: 0.5rem;
+  margin: -0.875rem -0.875rem 0;
+  padding: 0.3rem 0.5rem;
+  border-bottom: 1px solid var(--i18n-border);
+  border-radius: 0.75rem 0.75rem 0 0;
+  background: color-mix(in srgb, Canvas 88%, currentColor 12%);
+}
+
+.i18n-message__source-location {
+  display: flex;
+  min-width: 0;
+  align-items: center;
+  gap: 0.4rem;
+  color: var(--i18n-muted);
+}
+
+.i18n-message__source-location > svg {
+  width: 1rem;
+  height: 1rem;
+  flex: 0 0 1rem;
+}
+
+.i18n-message__source-location > code {
+  min-width: 0;
+  overflow: hidden;
+  font-size: 0.75rem;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.i18n-message__source-count {
+  display: inline-grid;
+  min-width: 1.35rem;
+  height: 1.25rem;
+  flex: none;
+  place-items: center;
+  padding: 0 0.3rem;
+  border-radius: 999px;
+  background: color-mix(in srgb, Canvas 78%, currentColor 22%);
+  color: var(--i18n-muted);
+  font-variant-numeric: tabular-nums;
+  font-size: 0.6875rem;
+}
+
+.i18n-message-copy {
+  position: relative;
+  flex: none;
+}
+
+.i18n-message-panel button.i18n-message-copy__trigger {
+  width: auto;
+  min-height: 1.625rem;
+  gap: 0.15rem;
+  padding: 0 0.25rem;
+  border: 0;
+  background: transparent;
+}
+
+.i18n-message-panel button.i18n-message-copy__trigger:hover:not(:disabled),
+.i18n-message-panel button.i18n-message-copy__trigger[aria-expanded="true"] {
+  background: transparent;
+  color: color-mix(in srgb, currentColor 78%, AccentColor 22%);
+}
+
+.i18n-message-copy__trigger[data-state="copied"] {
+  color: #16a34a;
+}
+
+.i18n-message-copy__trigger[data-state="error"] {
+  color: #dc2626;
+}
+
+.i18n-message-copy__chevron {
+  width: 0.75rem;
+  height: 0.75rem;
+  transition: rotate 120ms ease;
+}
+
+.i18n-message-copy__trigger[aria-expanded="true"] .i18n-message-copy__chevron {
+  rotate: 180deg;
+}
+
+.i18n-message-copy__menu {
+  position: absolute;
+  z-index: 10;
+  top: calc(100% + 0.35rem);
+  right: 0;
+  display: grid;
+  width: min(18rem, calc(100vw - 4rem));
+  padding: 0.3rem;
+  border: 1px solid var(--i18n-border);
+  border-radius: 0.65rem;
+  background: Canvas;
+  box-shadow:
+    0 0.75rem 2.5rem rgb(0 0 0 / 14%),
+    0 0.125rem 0.5rem rgb(0 0 0 / 8%);
+}
+
+.i18n-message-panel .i18n-message-copy__menu > button {
+  width: 100%;
+  min-height: 2.25rem;
+  justify-content: flex-start;
+  padding: 0.45rem 0.55rem;
+  border: 0;
+  background: transparent;
+  text-align: start;
+}
+
+.i18n-message-panel .i18n-message-copy__menu > button:hover:not(:disabled),
+.i18n-message-panel .i18n-message-copy__menu > button:focus-visible {
+  outline: 2px solid AccentColor;
+  outline-offset: -2px;
+  background: color-mix(in srgb, Canvas 88%, currentColor 12%);
+}
+
+.i18n-message-panel .i18n-message-copy__menu > button:hover:not(:disabled) {
+  outline-color: transparent;
+}
+
+.i18n-message-copy__menu > button:has(code) {
+  display: grid;
+  justify-items: start;
+  gap: 0.1rem;
+}
+
+.i18n-message-copy__menu button code {
+  display: block;
+  width: 100%;
+  overflow: hidden;
+  color: var(--i18n-muted);
+  font-size: 0.6875rem;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.i18n-message-copy__separator {
+  height: 1px;
+  margin: 0.3rem -0.3rem;
+  background: var(--i18n-border);
+}
+
+.i18n-message-copy__label {
+  padding: 0.35rem 0.55rem 0.2rem;
+  color: var(--i18n-muted);
+  font-size: 0.6875rem;
+  font-weight: 650;
+  letter-spacing: 0.04em;
+  text-transform: uppercase;
+}
+
+.i18n-message-copy__feedback {
+  position: absolute;
+  z-index: 11;
+  top: calc(100% + 0.35rem);
+  right: 0;
+  display: inline-flex;
+  min-height: 1.875rem;
+  align-items: center;
+  gap: 0.35rem;
+  padding: 0.35rem 0.55rem;
+  border: 1px solid var(--i18n-border);
+  border-radius: 0.5rem;
+  background: Canvas;
+  box-shadow:
+    0 0.5rem 1.5rem rgb(0 0 0 / 12%),
+    0 0.125rem 0.375rem rgb(0 0 0 / 7%);
+  color: var(--i18n-muted);
+  font-size: 0.75rem;
+  font-weight: 600;
+  pointer-events: none;
+  white-space: nowrap;
+  animation: i18n-message-copy-feedback-in 140ms ease-out;
+}
+
+.i18n-message-copy__feedback[data-state="copied"] {
+  color: #15803d;
+}
+
+.i18n-message-copy__feedback[data-state="error"] {
+  color: #dc2626;
+}
+
+.i18n-message-copy__feedback > svg {
+  width: 0.875rem;
+  height: 0.875rem;
+}
+
+@keyframes i18n-message-copy-feedback-in {
+  from {
+    opacity: 0;
+    transform: translateY(-0.2rem) scale(0.96);
+  }
+
+  to {
+    opacity: 1;
+    transform: translateY(0) scale(1);
+  }
+}
+
 .i18n-message__header,
 .i18n-message__actions,
 .i18n-message__meta {
@@ -118,20 +778,31 @@
 
 .i18n-message__header {
   justify-content: space-between;
+  min-width: 0;
 }
 
 .i18n-message__id {
+  min-width: 0;
   overflow-wrap: anywhere;
+  color: color-mix(in srgb, CanvasText 88%, transparent);
   font-size: 0.8125rem;
 }
 
-.i18n-message__catalog {
-  color: var(--i18n-muted);
-  font-size: 0.75rem;
+.i18n-message__content {
+  display: grid;
+  grid-template-columns: minmax(10rem, 0.7fr) minmax(14rem, 1.3fr);
+  gap: 0.75rem;
+  align-items: start;
+}
+
+.i18n-message__source {
+  min-width: 0;
+  padding: 0.45rem 0.25rem;
 }
 
 .i18n-message__source p {
-  margin: 0.25rem 0 0;
+  margin: 0.3rem 0 0;
+  overflow-wrap: anywhere;
   line-height: 1.5;
 }
 
@@ -141,6 +812,9 @@
 }
 
 .i18n-message__label {
+  display: inline-flex;
+  align-items: baseline;
+  gap: 0.4rem;
   color: var(--i18n-muted);
   font-size: 0.75rem;
   font-weight: 600;
@@ -148,24 +822,65 @@
   text-transform: uppercase;
 }
 
+.i18n-message__label code {
+  padding: 0.05rem 0.3rem;
+  border-radius: 0.25rem;
+  background: color-mix(in srgb, Canvas 80%, currentColor 20%);
+  color: color-mix(in srgb, CanvasText 76%, transparent);
+  font-size: 0.6875rem;
+  font-weight: 500;
+  letter-spacing: normal;
+  text-transform: none;
+}
+
 .i18n-message__meta {
+  min-width: 0;
   flex-wrap: wrap;
   color: var(--i18n-muted);
   font-size: 0.75rem;
 }
 
+.i18n-message__meta span {
+  min-width: 0;
+  max-width: 100%;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
 .i18n-message__actions {
   justify-content: flex-end;
+  min-height: 2.25rem;
 }
 
 .i18n-message__status {
   margin-inline-end: auto;
   color: var(--i18n-muted);
+  display: inline-flex;
+  align-items: center;
+  gap: 0.35rem;
   font-size: 0.75rem;
 }
 
+.i18n-message__status[data-state="saved"] {
+  color: #16a34a;
+}
+
+.i18n-message__status[data-state="error"] {
+  color: #dc2626;
+}
+
+.i18n-message__status svg {
+  width: 0.875rem;
+  height: 0.875rem;
+}
+
 .i18n-message-panel button {
+  display: inline-flex;
   min-height: 2.25rem;
+  align-items: center;
+  justify-content: center;
+  gap: 0.4rem;
   padding: 0 0.8rem;
   border: 1px solid var(--i18n-border);
   border-radius: 0.55rem;
@@ -173,10 +888,29 @@
   cursor: pointer;
 }
 
+.i18n-message-panel button svg {
+  width: 0.95rem;
+  height: 0.95rem;
+}
+
 .i18n-message-panel button:hover:not(:disabled) {
   background: color-mix(in srgb, Canvas 90%, currentColor 10%);
 }
 
+.i18n-message-panel button.i18n-message__save {
+  border-color: var(--i18n-devtool-primary, #2563eb);
+  background: var(--i18n-devtool-primary, #2563eb);
+  color: var(--i18n-devtool-panel, #fff);
+}
+
+.i18n-message-panel button.i18n-message__save:hover:not(:disabled) {
+  background: color-mix(
+    in srgb,
+    var(--i18n-devtool-primary, #2563eb) 86%,
+    black
+  );
+}
+
 .i18n-message-panel button:disabled {
   cursor: default;
   opacity: 0.45;
@@ -215,4 +949,33 @@
   .i18n-message-panel__search {
     flex-basis: 100%;
   }
+
+  .i18n-message-package__counts span {
+    padding-inline: 0.35rem;
+  }
+
+  .i18n-message__content {
+    grid-template-columns: minmax(0, 1fr);
+    gap: 0.5rem;
+  }
+
+  .i18n-message__source {
+    padding-bottom: 0.1rem;
+  }
+}
+
+@media (max-width: 52rem) {
+  .i18n-message-workspace {
+    grid-template-columns: minmax(0, 1fr);
+  }
+
+  .i18n-message-navigation {
+    max-height: 18rem;
+    border-right: 0;
+    border-bottom: 1px solid var(--i18n-border);
+  }
+
+  .i18n-message-workspace__editor {
+    min-height: 20rem;
+  }
 }

packages/i18n/src/devtool/message-panel.test.tsx

@@ -1,8 +1,15 @@
 // @vitest-environment jsdom
 
 import * as React from "react"
-import { fireEvent, render, screen, waitFor } from "@testing-library/react"
-import { describe, expect, it, vi } from "vitest"
+import {
+  act,
+  cleanup,
+  fireEvent,
+  render,
+  screen,
+  waitFor,
+} from "@testing-library/react"
+import { afterEach, describe, expect, it, vi } from "vitest"
 
 import { MessagePanel } from "./message-panel"
 import type { DevtoolMessage, MessageRepository } from "./types"
@@ -10,14 +17,23 @@ import type { DevtoolMessage, MessageRepository } from "./types"
 const message: DevtoolMessage = {
   catalog: "messages",
   comments: ["Navigation title"],
+  editable: true,
   id: "navigation.home",
   missing: true,
   obsolete: false,
   origins: [{ file: "src/navigation.tsx", line: 12 }],
+  packageName: "@workspace/ui",
   source: "Home",
+  sourceLocale: "en",
   translation: "",
 }
 
+afterEach(() => {
+  cleanup()
+  vi.restoreAllMocks()
+  Reflect.deleteProperty(window.navigator, "clipboard")
+})
+
 describe("MessagePanel", () => {
   it("loads, filters, and saves messages through the repository", async () => {
     const updateMessage = vi.fn(async (input) => ({
@@ -33,10 +49,13 @@ describe("MessagePanel", () => {
     render(<MessagePanel locale="zh-Hans" repository={repository} />)
 
     expect(await screen.findByText("navigation.home")).toBeTruthy()
+    expect(screen.getByRole("region", { name: "@workspace/ui" })).toBeTruthy()
 
     const textarea = screen.getByLabelText(
       "navigation.home translation for zh-Hans"
     )
+    expect(screen.queryByRole("button", { name: "Save" })).toBeNull()
+
     fireEvent.change(textarea, { target: { value: "首页" } })
     fireEvent.click(screen.getByRole("button", { name: "Save" }))
 
@@ -50,11 +69,323 @@ describe("MessagePanel", () => {
     })
     expect(await screen.findByText("Saved")).toBeTruthy()
 
+    const missingOnly = screen.getByRole("checkbox", {
+      name: "Missing only",
+    })
+    const checkboxIcon = missingOnly.nextElementSibling
+
+    expect(checkboxIcon?.querySelector("svg")).toBeTruthy()
+    fireEvent.click(missingOnly)
+    expect(missingOnly).toHaveProperty("checked", true)
+
     fireEvent.change(screen.getByPlaceholderText("Search messages"), {
       target: { value: "does not exist" },
     })
+    expect(screen.getByText("No messages match your search.")).toBeTruthy()
+  })
+
+  it("keeps the current content visible during a background refresh", async () => {
+    let resolveRefresh:
+      ((messages: readonly DevtoolMessage[]) => void) | undefined
+    const repository: MessageRepository = {
+      getMessages: vi
+        .fn()
+        .mockResolvedValueOnce([message])
+        .mockImplementationOnce(
+          () =>
+            new Promise<readonly DevtoolMessage[]>((resolve) => {
+              resolveRefresh = resolve
+            })
+        ),
+      updateMessage: vi.fn(),
+    }
+    const view = render(
+      <MessagePanel locale="zh-Hans" refreshToken={0} repository={repository} />
+    )
+
+    expect(await screen.findByText("navigation.home")).toBeTruthy()
+    fireEvent.change(screen.getByPlaceholderText("Search messages"), {
+      target: { value: "home" },
+    })
+
+    view.rerender(
+      <MessagePanel locale="zh-Hans" refreshToken={1} repository={repository} />
+    )
+
+    expect(screen.queryByText("Loading zh-Hans…")).toBeNull()
+    expect(screen.getByText("navigation.home")).toBeTruthy()
+    expect(screen.getByPlaceholderText("Search messages")).toHaveProperty(
+      "value",
+      "home"
+    )
+
+    await act(async () => {
+      resolveRefresh?.([])
+    })
+
     expect(
-      screen.getByText("No messages match the current filters.")
+      await screen.findByText(
+        "No messages are available yet. Extract the project to populate this catalog."
+      )
     ).toBeTruthy()
   })
+
+  it("collapses a package without discarding an unsaved translation", async () => {
+    const repository: MessageRepository = {
+      getMessages: vi.fn(async () => [message]),
+      updateMessage: vi.fn(),
+    }
+
+    render(<MessagePanel locale="zh-Hans" repository={repository} />)
+
+    const textarea = await screen.findByLabelText(
+      "navigation.home translation for zh-Hans"
+    )
+    fireEvent.change(textarea, { target: { value: "未保存的首页" } })
+
+    const collapseButton = screen.getByRole("button", {
+      name: "Collapse @workspace/ui",
+    })
+    const packageContent = screen
+      .getByRole("region", { name: "@workspace/ui" })
+      .querySelector(".i18n-message-package__messages")
+    fireEvent.click(collapseButton)
+
+    expect(collapseButton.getAttribute("aria-expanded")).toBe("false")
+    expect(textarea).toHaveProperty("value", "未保存的首页")
+    expect(packageContent).toHaveProperty("hidden", true)
+
+    fireEvent.click(
+      screen.getByRole("button", {
+        name: "Expand @workspace/ui",
+      })
+    )
+
+    expect(textarea).toHaveProperty("value", "未保存的首页")
+    expect(packageContent).toHaveProperty("hidden", false)
+  })
+
+  it("shows the application package even when it has no messages", async () => {
+    const repository: MessageRepository = {
+      getMessages: vi.fn(async () => [message]),
+      getPackages: vi.fn(async () => [
+        {
+          kind: "application" as const,
+          name: "web",
+        },
+      ]),
+      updateMessage: vi.fn(),
+    }
+
+    render(<MessagePanel locale="zh-Hans" repository={repository} />)
+
+    expect(await screen.findByRole("region", { name: "web" })).toBeTruthy()
+    expect(
+      screen
+        .getAllByRole("region")
+        .map((region) => region.getAttribute("aria-label"))
+        .filter((label) => label !== "Message editor")
+    ).toEqual(["web", "@workspace/ui"])
+    expect(
+      screen.getByText("No messages were found in this package.")
+    ).toBeTruthy()
+    expect(screen.getByRole("region", { name: "web" }).textContent).toContain(
+      "0 messages"
+    )
+  })
+
+  it("builds an ID tree and preserves visited editor drafts", async () => {
+    const activeMessage: DevtoolMessage = {
+      ...message,
+      id: "blocks.appearance.active",
+      source: "Active",
+    }
+    const darkMessage: DevtoolMessage = {
+      ...message,
+      id: "blocks.theme.dark",
+      source: "Dark",
+    }
+    const repository: MessageRepository = {
+      getMessages: vi.fn(async () => [activeMessage, darkMessage]),
+      updateMessage: vi.fn(),
+    }
+
+    render(<MessagePanel locale="zh-Hans" repository={repository} />)
+
+    const activeEditor = await screen.findByLabelText(
+      "blocks.appearance.active translation for zh-Hans"
+    )
+
+    expect(screen.getByTitle("blocks")).toBeTruthy()
+    expect(screen.getByTitle("blocks.appearance")).toBeTruthy()
+    expect(screen.getByTitle("blocks.theme")).toBeTruthy()
+    expect(screen.queryByTitle("blocks.appearance.active")).toBeNull()
+    expect(screen.queryByTitle("blocks.theme.dark")).toBeNull()
+    fireEvent.change(activeEditor, { target: { value: "使用中" } })
+    fireEvent.click(screen.getByTitle("blocks.theme"))
+
+    expect(
+      await screen.findByLabelText("blocks.theme.dark translation for zh-Hans")
+    ).toBeTruthy()
+
+    fireEvent.click(screen.getByTitle("blocks.appearance"))
+    expect(
+      await screen.findByLabelText(
+        "blocks.appearance.active translation for zh-Hans"
+      )
+    ).toHaveProperty("value", "使用中")
+  })
+
+  it("uses the final ID segment as right-pane content instead of a tree leaf", async () => {
+    const parentMessage: DevtoolMessage = {
+      ...message,
+      id: "a.b",
+      source: "B",
+    }
+    const childMessage: DevtoolMessage = {
+      ...message,
+      id: "a.b.c",
+      source: "C",
+    }
+    const repository: MessageRepository = {
+      getMessages: vi.fn(async () => [parentMessage, childMessage]),
+      updateMessage: vi.fn(),
+    }
+
+    render(<MessagePanel locale="zh-Hans" repository={repository} />)
+
+    expect(
+      await screen.findByLabelText("a.b translation for zh-Hans")
+    ).toBeTruthy()
+    expect(screen.getByTitle("a")).toBeTruthy()
+    expect(screen.getByTitle("a.b")).toBeTruthy()
+    expect(screen.queryByTitle("a.b.c")).toBeNull()
+    expect(screen.queryByLabelText("a.b.c translation for zh-Hans")).toBeNull()
+
+    fireEvent.click(screen.getByTitle("a.b"))
+
+    expect(
+      await screen.findByLabelText("a.b.c translation for zh-Hans")
+    ).toBeTruthy()
+  })
+
+  it("resets an unsaved translation to the repository value", async () => {
+    const updateMessage = vi.fn()
+    const repository: MessageRepository = {
+      getMessages: vi.fn(async () => [message]),
+      updateMessage,
+    }
+
+    render(<MessagePanel locale="zh-Hans" repository={repository} />)
+
+    const textarea = await screen.findByLabelText(
+      "navigation.home translation for zh-Hans"
+    )
+
+    expect(screen.getByText("src/navigation.tsx:12")).toBeTruthy()
+    fireEvent.change(textarea, { target: { value: "未保存的首页" } })
+    fireEvent.click(screen.getByRole("button", { name: "Reset" }))
+
+    expect(textarea).toHaveProperty("value", "")
+    expect(screen.queryByRole("button", { name: "Save" })).toBeNull()
+    expect(updateMessage).not.toHaveBeenCalled()
+  })
+
+  it("keeps application source messages read-only", async () => {
+    const updateMessage = vi.fn()
+    const repository: MessageRepository = {
+      getMessages: vi.fn(async () => [
+        {
+          ...message,
+          editable: false,
+          missing: false,
+          packageName: "test-app",
+          translation: "Home",
+        },
+      ]),
+      getPackages: vi.fn(async () => [
+        { kind: "application" as const, name: "test-app" },
+      ]),
+      updateMessage,
+    }
+
+    render(<MessagePanel locale="en" repository={repository} />)
+
+    const textarea = await screen.findByLabelText(
+      "navigation.home translation for en"
+    )
+
+    expect(textarea).toHaveProperty("readOnly", true)
+    fireEvent.change(textarea, { target: { value: "Customized home" } })
+
+    expect(textarea).toHaveProperty("value", "Home")
+    expect(screen.queryByRole("button", { name: "Save" })).toBeNull()
+    expect(updateMessage).not.toHaveBeenCalled()
+  })
+
+  it("copies an explicitly selected message value", async () => {
+    const writeText = vi.fn(async () => undefined)
+    Object.defineProperty(window.navigator, "clipboard", {
+      configurable: true,
+      value: { writeText },
+    })
+    const repository: MessageRepository = {
+      getMessages: vi.fn(async () => [message]),
+      updateMessage: vi.fn(),
+    }
+
+    render(<MessagePanel locale="zh-Hans" repository={repository} />)
+
+    await screen.findByText("navigation.home")
+    fireEvent.click(
+      screen.getByRole("button", {
+        name: "Choose content to copy",
+      })
+    )
+
+    expect(
+      screen.getByRole("menuitem", { name: "Copy current translation" })
+    ).toHaveProperty("disabled", true)
+    expect(
+      screen.getByRole("menuitem", {
+        name: /Copy source location/,
+      })
+    ).toBeTruthy()
+
+    fireEvent.click(
+      screen.getByRole("menuitem", {
+        name: "Copy message ID",
+      })
+    )
+
+    await waitFor(() => {
+      expect(writeText).toHaveBeenCalledWith("navigation.home")
+    })
+    expect(screen.getByRole("button", { name: "Copied" })).toBeTruthy()
+    expect(screen.queryByRole("menu")).toBeNull()
+    expect(screen.getByRole("status").textContent).toContain("Copied")
+  })
+
+  it("distinguishes an empty catalog from a fully translated catalog", async () => {
+    const translatedMessage = {
+      ...message,
+      missing: false,
+      translation: "首页",
+    }
+    const repository: MessageRepository = {
+      getMessages: vi.fn(async () => [translatedMessage]),
+      updateMessage: vi.fn(),
+    }
+
+    render(<MessagePanel locale="zh-Hans" repository={repository} />)
+
+    expect(await screen.findByText("navigation.home")).toBeTruthy()
+    fireEvent.click(
+      screen.getByRole("checkbox", {
+        name: "Missing only",
+      })
+    )
+
+    expect(screen.getByText("All messages have translations.")).toBeTruthy()
+  })
 })

packages/i18n/src/devtool/message-panel.tsx

@@ -1,17 +1,40 @@
 import * as React from "react"
+import {
+  CircleAlertIcon,
+  CheckIcon,
+  FileCode2Icon,
+  RotateCcwIcon,
+  SaveIcon,
+  SearchIcon,
+} from "lucide-react"
 
+import { useDevtoolLocalization } from "./devtool-localization"
+import { formatMessageOrigin, MessageCopyMenu } from "./message-copy-menu"
+import {
+  createMessageNavigationModel,
+  MessageNavigation,
+} from "./message-navigation"
 import { useOptionalMessageRepository } from "./message-repository"
-import type { DevtoolMessage, MessageRepository } from "./types"
+import type {
+  DevtoolMessage,
+  DevtoolMessagePackage,
+  MessageRepository,
+} from "./types"
 import "./message-panel.css"
 
 type LoadState =
   | { status: "loading" }
   | { error: Error; status: "error" }
-  | { messages: readonly DevtoolMessage[]; status: "ready" }
+  | {
+      messages: readonly DevtoolMessage[]
+      packages: readonly DevtoolMessagePackage[]
+      status: "ready"
+    }
 
 export interface MessagePanelProps {
   className?: string
   locale: string
+  refreshToken?: unknown
   repository?: MessageRepository
 }
 
@@ -20,6 +43,7 @@ function getMessageSearchText(message: DevtoolMessage) {
     message.id,
     message.source,
     message.translation,
+    message.packageName,
     ...message.comments,
     ...message.origins.map((origin) => origin.file),
   ]
@@ -36,14 +60,23 @@ function MessageEditor({
   message: DevtoolMessage
   onSave: (translation: string) => Promise<void>
 }) {
+  const { messages } = useDevtoolLocalization()
   const [translation, setTranslation] = React.useState(message.translation)
   const [saveState, setSaveState] = React.useState<
     "idle" | "saving" | "saved" | "error"
   >("idle")
+  const repositoryTranslationRef = React.useRef(message.translation)
   const submittedTranslationRef = React.useRef<string | undefined>(undefined)
+  const isEditable = message.editable
   const isDirty = translation !== message.translation
+  const primaryOrigin = message.origins[0]
 
   React.useEffect(() => {
+    if (repositoryTranslationRef.current === message.translation) {
+      return
+    }
+
+    repositoryTranslationRef.current = message.translation
     setTranslation(message.translation)
 
     if (submittedTranslationRef.current === message.translation) {
@@ -54,8 +87,17 @@ function MessageEditor({
     setSaveState("idle")
   }, [message.translation])
 
+  React.useEffect(() => {
+    if (saveState !== "saved") {
+      return
+    }
+
+    const timeout = window.setTimeout(() => setSaveState("idle"), 1600)
+    return () => window.clearTimeout(timeout)
+  }, [saveState])
+
   const handleSave = async () => {
-    if (!isDirty || saveState === "saving") {
+    if (!isEditable || !isDirty || saveState === "saving") {
       return
     }
 
@@ -71,69 +113,152 @@ function MessageEditor({
     }
   }
 
+  const handleReset = () => {
+    submittedTranslationRef.current = undefined
+    setTranslation(message.translation)
+    setSaveState("idle")
+  }
+
   return (
     <article
       className="i18n-message"
+      data-dirty={isDirty || undefined}
       data-missing={message.missing || undefined}
       data-obsolete={message.obsolete || undefined}
+      data-readonly={!isEditable || undefined}
+      data-save-state={saveState}
     >
-      <header className="i18n-message__header">
-        <code className="i18n-message__id">{message.id}</code>
-        <span className="i18n-message__catalog">{message.catalog}</span>
+      <header className="i18n-message__source-toolbar">
+        <div
+          className="i18n-message__source-location"
+          title={
+            primaryOrigin
+              ? `${primaryOrigin.file}${primaryOrigin.line ? `:${primaryOrigin.line}` : ""}`
+              : messages.messagePanel.unknownSourceLocation
+          }
+        >
+          <FileCode2Icon aria-hidden="true" />
+          <code>
+            {primaryOrigin
+              ? formatMessageOrigin(primaryOrigin)
+              : messages.messagePanel.unknownSourceLocation}
+          </code>
+          {message.origins.length > 1 && (
+            <span
+              aria-label={messages.messagePanel.additionalSourceLocations(
+                message.origins.length - 1
+              )}
+              className="i18n-message__source-count"
+            >
+              +{message.origins.length - 1}
+            </span>
+          )}
+        </div>
+        <MessageCopyMenu message={message} translation={translation} />
       </header>
 
-      <div className="i18n-message__source">
-        <span className="i18n-message__label">Source</span>
-        <p>{message.source || message.id}</p>
+      <div className="i18n-message__header">
+        <code className="i18n-message__id">{message.id}</code>
       </div>
 
-      <label className="i18n-message__field">
-        <span className="i18n-message__label">{locale}</span>
-        <textarea
-          aria-label={`${message.id} translation for ${locale}`}
-          value={translation}
-          onChange={(event) => {
-            submittedTranslationRef.current = undefined
-            setTranslation(event.target.value)
-            setSaveState("idle")
-          }}
-          onKeyDown={(event) => {
-            if ((event.metaKey || event.ctrlKey) && event.key === "Enter") {
-              event.preventDefault()
-              void handleSave()
-            }
-          }}
-        />
-      </label>
+      <div className="i18n-message__content">
+        <div className="i18n-message__source">
+          <span className="i18n-message__label">
+            {messages.messagePanel.source}
+            <code>{message.sourceLocale}</code>
+          </span>
+          <p dir="auto" lang={message.sourceLocale}>
+            {message.source || message.id}
+          </p>
+        </div>
 
-      {(message.comments.length > 0 || message.origins.length > 0) && (
+        <label className="i18n-message__field">
+          <span className="i18n-message__label">
+            {messages.messagePanel.translation}
+            <code>{locale}</code>
+          </span>
+          <textarea
+            aria-label={messages.messagePanel.translationFor(
+              message.id,
+              locale
+            )}
+            dir="auto"
+            lang={locale}
+            rows={1}
+            readOnly={!isEditable}
+            value={translation}
+            onChange={(event) => {
+              if (!isEditable) {
+                return
+              }
+
+              submittedTranslationRef.current = undefined
+              setTranslation(event.target.value)
+              setSaveState("idle")
+            }}
+            onKeyDown={(event) => {
+              if (
+                isEditable &&
+                (event.metaKey || event.ctrlKey) &&
+                event.key === "Enter"
+              ) {
+                event.preventDefault()
+                void handleSave()
+              }
+            }}
+          />
+        </label>
+      </div>
+
+      {message.comments.length > 0 && (
         <footer className="i18n-message__meta">
           {message.comments.map((comment) => (
             <span key={comment}>{comment}</span>
           ))}
-          {message.origins.map((origin) => (
-            <span key={`${origin.file}:${origin.line ?? ""}`}>
-              {origin.file}
-              {origin.line ? `:${origin.line}` : ""}
-            </span>
-          ))}
         </footer>
       )}
 
-      <div className="i18n-message__actions">
-        <span aria-live="polite" className="i18n-message__status">
-          {saveState === "saving" && "Saving…"}
-          {saveState === "saved" && "Saved"}
-          {saveState === "error" && "Could not save"}
-        </span>
-        <button
-          type="button"
-          disabled={!isDirty || saveState === "saving"}
-          onClick={() => void handleSave()}
-        >
-          Save
-        </button>
-      </div>
+      {(isDirty || saveState !== "idle") && (
+        <div className="i18n-message__actions">
+          <span
+            aria-live="polite"
+            className="i18n-message__status"
+            data-state={saveState}
+          >
+            {saveState === "saved" && (
+              <>
+                <CheckIcon aria-hidden="true" />
+                {messages.messagePanel.saved}
+              </>
+            )}
+            {saveState === "error" && (
+              <>
+                <CircleAlertIcon aria-hidden="true" />
+                {messages.messagePanel.couldNotSave}
+              </>
+            )}
+          </span>
+          {isDirty && (
+            <>
+              <button type="button" onClick={handleReset}>
+                <RotateCcwIcon aria-hidden="true" />
+                {messages.messagePanel.reset}
+              </button>
+              <button
+                type="button"
+                className="i18n-message__save"
+                disabled={saveState === "saving"}
+                onClick={() => void handleSave()}
+              >
+                <SaveIcon aria-hidden="true" />
+                {saveState === "saving"
+                  ? messages.messagePanel.saving
+                  : messages.messagePanel.save}
+              </button>
+            </>
+          )}
+        </div>
+      )}
     </article>
   )
 }
@@ -141,57 +266,82 @@ function MessageEditor({
 export function MessagePanel({
   className,
   locale,
+  refreshToken,
   repository: repositoryProp,
 }: MessagePanelProps) {
   const repositoryFromContext = useOptionalMessageRepository()
   const repository = repositoryProp ?? repositoryFromContext
+  const { messages } = useDevtoolLocalization()
   const [state, setState] = React.useState<LoadState>({ status: "loading" })
   const [query, setQuery] = React.useState("")
   const [missingOnly, setMissingOnly] = React.useState(false)
+  const [selectedNavigationKey, setSelectedNavigationKey] =
+    React.useState<string>()
+  const [openedNavigationKeys, setOpenedNavigationKeys] = React.useState<
+    readonly string[]
+  >([])
   const loadRequestRef = React.useRef(0)
+  const previousLoadContextRef = React.useRef<{
+    locale: string
+    repository: MessageRepository | null
+  } | null>(null)
 
-  const loadMessages = React.useCallback(async () => {
-    const requestId = loadRequestRef.current + 1
-    loadRequestRef.current = requestId
+  const loadMessages = React.useCallback(
+    async ({ preserveContent = false } = {}) => {
+      const requestId = loadRequestRef.current + 1
+      loadRequestRef.current = requestId
 
-    if (!repository) {
-      setState({
-        error: new Error(
-          "MessagePanel requires a MessageRepositoryProvider or a repository prop."
-        ),
-        status: "error",
-      })
-      return
-    }
-
-    setState({ status: "loading" })
-
-    try {
-      const messages = await repository.getMessages(locale)
-
-      if (requestId !== loadRequestRef.current) {
+      if (!repository) {
+        setState({
+          error: new Error(
+            "MessagePanel requires a MessageRepositoryProvider or a repository prop."
+          ),
+          status: "error",
+        })
         return
       }
 
-      setState({ messages, status: "ready" })
-    } catch (error) {
-      if (requestId !== loadRequestRef.current) {
-        return
+      if (!preserveContent) {
+        setState({ status: "loading" })
       }
 
-      setState({
-        error:
-          error instanceof Error
-            ? error
-            : new Error("Unable to load messages."),
-        status: "error",
-      })
-    }
-  }, [locale, repository])
+      try {
+        const [loadedMessages, packages] = await Promise.all([
+          repository.getMessages(locale),
+          repository.getPackages?.() ?? Promise.resolve([]),
+        ])
+
+        if (requestId !== loadRequestRef.current) {
+          return
+        }
+
+        setState({ messages: loadedMessages, packages, status: "ready" })
+      } catch (error) {
+        if (requestId !== loadRequestRef.current) {
+          return
+        }
+
+        setState({
+          error:
+            error instanceof Error
+              ? error
+              : new Error("Unable to load messages."),
+          status: "error",
+        })
+      }
+    },
+    [locale, repository]
+  )
 
   React.useEffect(() => {
-    void loadMessages()
-  }, [loadMessages])
+    const previousLoadContext = previousLoadContextRef.current
+    const preserveContent =
+      previousLoadContext?.locale === locale &&
+      previousLoadContext.repository === repository
+
+    previousLoadContextRef.current = { locale, repository }
+    void loadMessages({ preserveContent })
+  }, [loadMessages, locale, refreshToken, repository])
 
   const visibleMessages = React.useMemo(() => {
     if (state.status !== "ready") {
@@ -211,6 +361,92 @@ export function MessagePanel({
       )
     })
   }, [missingOnly, query, state])
+  const visiblePackageGroups = React.useMemo(() => {
+    const groups = new Map<
+      string,
+      {
+        kind: DevtoolMessagePackage["kind"]
+        messages: DevtoolMessage[]
+      }
+    >()
+
+    if (state.status === "ready" && query.trim().length === 0 && !missingOnly) {
+      for (const packageDefinition of state.packages) {
+        groups.set(packageDefinition.name, {
+          kind: packageDefinition.kind,
+          messages: [],
+        })
+      }
+    }
+
+    for (const message of visibleMessages) {
+      const group = groups.get(message.packageName)
+
+      if (group) {
+        group.messages.push(message)
+      } else {
+        groups.set(message.packageName, {
+          kind: "dependency",
+          messages: [message],
+        })
+      }
+    }
+
+    return Array.from(groups, ([packageName, group]) => ({
+      kind: group.kind,
+      messages: group.messages,
+      missingCount: group.messages.filter((message) => message.missing).length,
+      packageName,
+    })).sort(
+      (left, right) =>
+        Number(right.kind === "application") -
+          Number(left.kind === "application") ||
+        left.packageName.localeCompare(right.packageName)
+    )
+  }, [missingOnly, query, state, visibleMessages])
+  const navigationModel = React.useMemo(
+    () => createMessageNavigationModel(visiblePackageGroups),
+    [visiblePackageGroups]
+  )
+  const navigationSelections = React.useMemo(
+    () => Array.from(navigationModel.selections.values()),
+    [navigationModel]
+  )
+  const activeSelection =
+    (selectedNavigationKey
+      ? navigationModel.selections.get(selectedNavigationKey)
+      : undefined) ??
+    navigationSelections.find((selection) => selection.messages.length > 0) ??
+    navigationSelections[0]
+  const activeNavigationKey = activeSelection?.key
+
+  React.useEffect(() => {
+    if (!activeNavigationKey) {
+      return
+    }
+
+    setOpenedNavigationKeys((current) =>
+      current.includes(activeNavigationKey)
+        ? current
+        : [...current, activeNavigationKey]
+    )
+  }, [activeNavigationKey])
+
+  const openedSelections = React.useMemo(() => {
+    const activeKeys =
+      activeNavigationKey && !openedNavigationKeys.includes(activeNavigationKey)
+        ? [...openedNavigationKeys, activeNavigationKey]
+        : openedNavigationKeys
+
+    return activeKeys.flatMap((key) => {
+      const selection = navigationModel.selections.get(key)
+      return selection ? [selection] : []
+    })
+  }, [activeNavigationKey, navigationModel, openedNavigationKeys])
+  const missingMessageCount =
+    state.status === "ready"
+      ? state.messages.filter((message) => message.missing).length
+      : 0
 
   const updateMessage = React.useCallback(
     async (message: DevtoolMessage, translation: string) => {
@@ -218,6 +454,10 @@ export function MessagePanel({
         throw new Error("No message repository is available.")
       }
 
+      if (!message.editable) {
+        throw new Error("This source message is read-only.")
+      }
+
       const updatedMessage = await repository.updateMessage({
         catalog: message.catalog,
         id: message.id,
@@ -237,6 +477,7 @@ export function MessagePanel({
               ? updatedMessage
               : item
           ),
+          packages: current.packages,
           status: "ready",
         }
       })
@@ -247,18 +488,39 @@ export function MessagePanel({
   const rootClassName = ["i18n-message-panel", className]
     .filter(Boolean)
     .join(" ")
+  const emptyStateMessage =
+    state.status !== "ready" || visibleMessages.length > 0
+      ? undefined
+      : state.messages.length === 0
+        ? messages.messagePanel.emptyCatalog
+        : query.trim().length > 0
+          ? messages.messagePanel.noSearchResults
+          : missingOnly
+            ? messages.messagePanel.noMissingMessages
+            : messages.messagePanel.noMatches
 
   return (
     <section className={rootClassName}>
       <header className="i18n-message-panel__toolbar">
         <label className="i18n-message-panel__search">
-          <span className="i18n-visually-hidden">Search messages</span>
+          <span className="i18n-visually-hidden">
+            {messages.messagePanel.search}
+          </span>
+          <SearchIcon aria-hidden="true" />
           <input
             type="search"
-            placeholder="Search messages"
+            placeholder={messages.messagePanel.search}
             value={query}
             onChange={(event) => setQuery(event.target.value)}
           />
+          {state.status === "ready" && (
+            <span
+              aria-hidden="true"
+              className="i18n-message-panel__result-count"
+            >
+              {visibleMessages.length}
+            </span>
+          )}
         </label>
         <label className="i18n-message-panel__toggle">
           <input
@@ -266,40 +528,83 @@ export function MessagePanel({
             checked={missingOnly}
             onChange={(event) => setMissingOnly(event.target.checked)}
           />
-          Missing only
+          <span aria-hidden="true" className="i18n-message-panel__checkbox">
+            <CheckIcon />
+          </span>
+          <span>{messages.messagePanel.missingOnly}</span>
+          {state.status === "ready" && (
+            <span aria-hidden="true" className="i18n-message-panel__count">
+              {missingMessageCount}
+            </span>
+          )}
         </label>
-        {state.status === "ready" && (
-          <output className="i18n-message-panel__count">
-            {visibleMessages.length} / {state.messages.length}
-          </output>
-        )}
       </header>
 
       {state.status === "loading" ? (
         <div className="i18n-message-panel__state" role="status">
-          Loading {locale}…
+          {messages.messagePanel.loading(locale)}
         </div>
       ) : state.status === "error" ? (
         <div className="i18n-message-panel__state" role="alert">
           <p>{state.error.message}</p>
           <button type="button" onClick={() => void loadMessages()}>
-            Retry
+            {messages.messagePanel.retry}
           </button>
         </div>
-      ) : visibleMessages.length === 0 ? (
-        <div className="i18n-message-panel__state">
-          No messages match the current filters.
-        </div>
+      ) : visiblePackageGroups.length === 0 ? (
+        <div className="i18n-message-panel__state">{emptyStateMessage}</div>
       ) : (
-        <div className="i18n-message-panel__list">
-          {visibleMessages.map((message) => (
-            <MessageEditor
-              key={`${message.catalog}:${message.id}`}
-              locale={locale}
-              message={message}
-              onSave={(translation) => updateMessage(message, translation)}
-            />
-          ))}
+        <div className="i18n-message-workspace">
+          <MessageNavigation
+            model={navigationModel}
+            selectedKey={activeNavigationKey}
+            onSelect={setSelectedNavigationKey}
+          />
+          <section
+            aria-label={messages.messagePanel.messageEditor}
+            className="i18n-message-workspace__editor"
+          >
+            {activeSelection ? (
+              openedSelections.map((selection) => {
+                return (
+                  <React.Activity
+                    key={selection.key}
+                    mode={
+                      selection.key === activeNavigationKey
+                        ? "visible"
+                        : "hidden"
+                    }
+                    name={selection.label}
+                  >
+                    {selection.messages.length > 0 ? (
+                      <div className="i18n-message-workspace__editor-list">
+                        {selection.messages.map((message) => (
+                          <MessageEditor
+                            key={`${message.catalog}:${message.id}`}
+                            locale={locale}
+                            message={message}
+                            onSave={(translation) =>
+                              updateMessage(message, translation)
+                            }
+                          />
+                        ))}
+                      </div>
+                    ) : (
+                      <div className="i18n-message-workspace__empty">
+                        {visibleMessages.length === 0
+                          ? messages.messagePanel.emptyPackage
+                          : messages.messagePanel.selectMessage}
+                      </div>
+                    )}
+                  </React.Activity>
+                )
+              })
+            ) : (
+              <div className="i18n-message-workspace__empty">
+                {messages.messagePanel.selectMessage}
+              </div>
+            )}
+          </section>
         </div>
       )}
     </section>

packages/i18n/src/devtool/types.ts

@@ -6,14 +6,22 @@ export interface MessageOrigin {
 export interface DevtoolMessage {
   catalog: string
   comments: readonly string[]
+  editable: boolean
   id: string
   missing: boolean
   obsolete: boolean
   origins: readonly MessageOrigin[]
+  packageName: string
   source: string
+  sourceLocale: string
   translation: string
 }
 
+export interface DevtoolMessagePackage {
+  kind: "application" | "dependency"
+  name: string
+}
+
 export interface UpdateMessageInput {
   catalog: string
   id: string
@@ -23,5 +31,6 @@ export interface UpdateMessageInput {
 
 export interface MessageRepository {
   getMessages(locale: string): Promise<readonly DevtoolMessage[]>
+  getPackages?(): Promise<readonly DevtoolMessagePackage[]>
   updateMessage(input: UpdateMessageInput): Promise<DevtoolMessage>
 }

packages/i18n/src/devtool/use-floating-trigger.ts

@@ -0,0 +1,451 @@
+import * as React from "react"
+
+type TriggerDockEdge = "bottom" | "left" | "right" | "top"
+
+interface TriggerPosition {
+  x: number
+  y: number
+}
+
+interface TriggerDockPlacement {
+  edge: TriggerDockEdge
+  offset: number | null
+}
+
+interface TriggerDragSession {
+  height: number
+  lastPosition: TriggerPosition
+  longPressed: boolean
+  longPressTimer: number | null
+  moved: boolean
+  offsetX: number
+  offsetY: number
+  pointerId: number
+  previousDockPlacement: TriggerDockPlacement | null
+  previousPosition: TriggerPosition | null
+  startX: number
+  startY: number
+  width: number
+}
+
+interface StoredTriggerPlacement {
+  dockPlacement: TriggerDockPlacement | null
+  position: TriggerPosition | null
+  version: 1
+}
+
+interface UseFloatingTriggerOptions {
+  storageKey?: string
+}
+
+const TRIGGER_DEFAULT_INSET = 16
+const TRIGGER_DOCK_THRESHOLD = 32
+const TRIGGER_DRAG_THRESHOLD = 4
+const TRIGGER_LONG_PRESS_DELAY = 400
+const DEFAULT_STORAGE_KEY = "i18n-devtool:floating-trigger-placement"
+const DEFAULT_PLACEMENT: StoredTriggerPlacement = {
+  dockPlacement: {
+    edge: "right",
+    offset: null,
+  },
+  position: null,
+  version: 1,
+}
+
+function clamp(value: number, minimum: number, maximum: number) {
+  return Math.min(Math.max(value, minimum), Math.max(minimum, maximum))
+}
+
+function isDockEdge(value: unknown): value is TriggerDockEdge {
+  return (
+    value === "bottom" ||
+    value === "left" ||
+    value === "right" ||
+    value === "top"
+  )
+}
+
+function parseStoredPlacement(value: string | null) {
+  if (!value) {
+    return null
+  }
+
+  try {
+    const parsed = JSON.parse(value) as Partial<StoredTriggerPlacement>
+    const rawDockPlacement = parsed.dockPlacement
+    const rawPosition = parsed.position
+    const dockPlacement =
+      rawDockPlacement === null
+        ? null
+        : rawDockPlacement &&
+            isDockEdge(rawDockPlacement.edge) &&
+            (rawDockPlacement.offset === null ||
+              (typeof rawDockPlacement.offset === "number" &&
+                Number.isFinite(rawDockPlacement.offset)))
+          ? {
+              edge: rawDockPlacement.edge,
+              offset: rawDockPlacement.offset,
+            }
+          : undefined
+    const position =
+      rawPosition === null
+        ? null
+        : rawPosition &&
+            typeof rawPosition.x === "number" &&
+            Number.isFinite(rawPosition.x) &&
+            typeof rawPosition.y === "number" &&
+            Number.isFinite(rawPosition.y)
+          ? { x: rawPosition.x, y: rawPosition.y }
+          : undefined
+
+    if (
+      parsed.version !== 1 ||
+      dockPlacement === undefined ||
+      position === undefined
+    ) {
+      return null
+    }
+
+    return {
+      dockPlacement,
+      position,
+      version: 1,
+    } satisfies StoredTriggerPlacement
+  } catch {
+    return null
+  }
+}
+
+function readStoredPlacement(storageKey: string) {
+  if (typeof window === "undefined") {
+    return null
+  }
+
+  try {
+    return parseStoredPlacement(window.localStorage.getItem(storageKey))
+  } catch {
+    return null
+  }
+}
+
+function resolveDockPlacement(
+  position: TriggerPosition,
+  width: number,
+  height: number
+): TriggerDockPlacement | null {
+  const distances: readonly [TriggerDockEdge, number][] = [
+    ["left", position.x],
+    ["right", window.innerWidth - position.x - width],
+    ["top", position.y],
+    ["bottom", window.innerHeight - position.y - height],
+  ]
+  const [edge, distance] = distances.reduce((nearest, candidate) =>
+    candidate[1] < nearest[1] ? candidate : nearest
+  )
+
+  if (distance > TRIGGER_DOCK_THRESHOLD) {
+    return null
+  }
+
+  return {
+    edge,
+    offset: edge === "left" || edge === "right" ? position.y : position.x,
+  }
+}
+
+function getTriggerStyle(
+  dockPlacement: TriggerDockPlacement | null,
+  position: TriggerPosition | null
+): React.CSSProperties | undefined {
+  if (!dockPlacement) {
+    return position
+      ? {
+          left: position.x,
+          top: position.y,
+        }
+      : undefined
+  }
+
+  const { edge, offset } = dockPlacement
+
+  if (edge === "left" || edge === "right") {
+    return {
+      [edge]: 0,
+      ...(offset === null
+        ? { bottom: TRIGGER_DEFAULT_INSET }
+        : { top: offset }),
+    }
+  }
+
+  return {
+    [edge]: 0,
+    ...(offset === null ? { right: TRIGGER_DEFAULT_INSET } : { left: offset }),
+  }
+}
+
+export function useFloatingTrigger({
+  storageKey = DEFAULT_STORAGE_KEY,
+}: UseFloatingTriggerOptions = {}) {
+  const [initialPlacement] = React.useState(
+    () => readStoredPlacement(storageKey) ?? DEFAULT_PLACEMENT
+  )
+  const triggerRef = React.useRef<HTMLButtonElement>(null)
+  const dragSessionRef = React.useRef<TriggerDragSession | null>(null)
+  const suppressClickRef = React.useRef(false)
+  const [dockPlacement, setDockPlacement] =
+    React.useState<TriggerDockPlacement | null>(initialPlacement.dockPlacement)
+  const [position, setPosition] = React.useState<TriggerPosition | null>(
+    initialPlacement.position
+  )
+  const [isDragging, setDragging] = React.useState(false)
+  const [isHovered, setHovered] = React.useState(false)
+  const [isFocused, setFocused] = React.useState(false)
+
+  React.useEffect(
+    () => () => {
+      const timer = dragSessionRef.current?.longPressTimer
+
+      if (timer !== null && timer !== undefined) {
+        window.clearTimeout(timer)
+      }
+    },
+    []
+  )
+
+  React.useLayoutEffect(() => {
+    const keepTriggerInViewport = () => {
+      const trigger = triggerRef.current
+
+      if (!trigger || trigger.hidden) {
+        return
+      }
+
+      const { width, height } = trigger.getBoundingClientRect()
+
+      setPosition((current) =>
+        current
+          ? {
+              x: clamp(current.x, 0, window.innerWidth - width),
+              y: clamp(current.y, 0, window.innerHeight - height),
+            }
+          : current
+      )
+      setDockPlacement((current) => {
+        if (!current || current.offset === null) {
+          return current
+        }
+
+        const maximumOffset =
+          current.edge === "left" || current.edge === "right"
+            ? window.innerHeight - height
+            : window.innerWidth - width
+        const offset = clamp(current.offset, 0, maximumOffset)
+
+        return offset === current.offset ? current : { ...current, offset }
+      })
+    }
+
+    keepTriggerInViewport()
+    window.addEventListener("resize", keepTriggerInViewport)
+    return () => window.removeEventListener("resize", keepTriggerInViewport)
+  }, [])
+
+  React.useEffect(() => {
+    if (isDragging) {
+      return
+    }
+
+    try {
+      window.localStorage.setItem(
+        storageKey,
+        JSON.stringify({
+          dockPlacement,
+          position,
+          version: 1,
+        } satisfies StoredTriggerPlacement)
+      )
+    } catch {
+      // Storage can be unavailable in privacy-restricted browser contexts.
+    }
+  }, [dockPlacement, isDragging, position, storageKey])
+
+  const handlePointerDown = (event: React.PointerEvent<HTMLButtonElement>) => {
+    if (event.button !== 0) {
+      return
+    }
+
+    const trigger = event.currentTarget
+    const rect = trigger.getBoundingClientRect()
+    const nextPosition = { x: rect.left, y: rect.top }
+
+    const session: TriggerDragSession = {
+      height: rect.height,
+      lastPosition: nextPosition,
+      longPressed: false,
+      longPressTimer: null,
+      moved: false,
+      offsetX: event.clientX - rect.left,
+      offsetY: event.clientY - rect.top,
+      pointerId: event.pointerId,
+      previousDockPlacement: dockPlacement,
+      previousPosition: position,
+      startX: event.clientX,
+      startY: event.clientY,
+      width: rect.width,
+    }
+    session.longPressTimer = window.setTimeout(() => {
+      if (dragSessionRef.current !== session || session.moved) {
+        return
+      }
+
+      session.longPressed = true
+      session.longPressTimer = null
+      setDragging(true)
+    }, TRIGGER_LONG_PRESS_DELAY)
+    dragSessionRef.current = session
+    trigger.setPointerCapture(event.pointerId)
+  }
+
+  const handlePointerMove = (event: React.PointerEvent<HTMLButtonElement>) => {
+    const session = dragSessionRef.current
+
+    if (!session || session.pointerId !== event.pointerId) {
+      return
+    }
+
+    let startedDragging = false
+
+    if (
+      !session.moved &&
+      Math.hypot(
+        event.clientX - session.startX,
+        event.clientY - session.startY
+      ) >= TRIGGER_DRAG_THRESHOLD
+    ) {
+      session.moved = true
+      startedDragging = true
+
+      if (session.longPressTimer !== null) {
+        window.clearTimeout(session.longPressTimer)
+        session.longPressTimer = null
+      }
+    }
+
+    if (!session.moved) {
+      return
+    }
+
+    event.preventDefault()
+    const nextPosition = {
+      x: clamp(
+        event.clientX - session.offsetX,
+        0,
+        window.innerWidth - session.width
+      ),
+      y: clamp(
+        event.clientY - session.offsetY,
+        0,
+        window.innerHeight - session.height
+      ),
+    }
+
+    session.lastPosition = nextPosition
+
+    if (startedDragging) {
+      setDockPlacement(null)
+      setDragging(true)
+    }
+
+    setPosition(nextPosition)
+  }
+
+  const finishPointerInteraction = (
+    event: React.PointerEvent<HTMLButtonElement>,
+    canceled: boolean
+  ) => {
+    const session = dragSessionRef.current
+
+    if (!session || session.pointerId !== event.pointerId) {
+      return
+    }
+
+    dragSessionRef.current = null
+    setDragging(false)
+
+    if (session.longPressTimer !== null) {
+      window.clearTimeout(session.longPressTimer)
+    }
+
+    if (event.currentTarget.hasPointerCapture(event.pointerId)) {
+      event.currentTarget.releasePointerCapture(event.pointerId)
+    }
+
+    if (canceled && session.moved) {
+      setDockPlacement(session.previousDockPlacement)
+      setPosition(session.previousPosition)
+      return
+    }
+
+    if (!session.moved) {
+      if (session.longPressed && !canceled) {
+        setFocused(false)
+
+        if (document.activeElement === event.currentTarget) {
+          event.currentTarget.blur()
+        }
+
+        suppressClickRef.current = true
+        window.setTimeout(() => {
+          suppressClickRef.current = false
+        }, 0)
+      }
+
+      return
+    }
+
+    setDockPlacement(
+      resolveDockPlacement(session.lastPosition, session.width, session.height)
+    )
+    setFocused(false)
+
+    if (document.activeElement === event.currentTarget) {
+      event.currentTarget.blur()
+    }
+
+    suppressClickRef.current = true
+    window.setTimeout(() => {
+      suppressClickRef.current = false
+    }, 0)
+  }
+
+  const isCollapsed =
+    dockPlacement !== null && !isDragging && !isHovered && !isFocused
+
+  return {
+    dockEdge: dockPlacement?.edge,
+    handleBlur: () => setFocused(false),
+    handleClick: () => {
+      if (suppressClickRef.current) {
+        suppressClickRef.current = false
+        return true
+      }
+
+      setFocused(false)
+      setHovered(false)
+      return false
+    },
+    handleFocus: () => setFocused(true),
+    handlePointerCancel: (event: React.PointerEvent<HTMLButtonElement>) =>
+      finishPointerInteraction(event, true),
+    handlePointerDown,
+    handlePointerEnter: () => setHovered(true),
+    handlePointerLeave: () => setHovered(false),
+    handlePointerMove,
+    handlePointerUp: (event: React.PointerEvent<HTMLButtonElement>) =>
+      finishPointerInteraction(event, false),
+    isCollapsed,
+    isDragging,
+    style: getTriggerStyle(dockPlacement, position),
+    triggerRef,
+  }
+}

packages/i18n/src/runtime/catalog-loader.ts

@@ -0,0 +1,12 @@
+import type { MessageCatalog } from "./types"
+
+/**
+ * This module is replaced by the @workspace/i18n Vite plugin.
+ */
+export function loadMessageCatalog(_locale: string): Promise<MessageCatalog> {
+  return Promise.reject(
+    new Error(
+      "The i18n catalog loader requires the @workspace/i18n Vite plugin."
+    )
+  )
+}

packages/i18n/src/runtime/catalogs.ts

@@ -0,0 +1,28 @@
+import type { MessageCatalogs } from "./types"
+
+export type MessageCatalogSource = MessageCatalogs | undefined
+
+/**
+ * Combines package and application catalogs by locale. Later sources override
+ * earlier ones, allowing an application to customize built-in translations.
+ */
+export function mergeMessageCatalogs(
+  ...sources: readonly MessageCatalogSource[]
+): MessageCatalogs {
+  const catalogs: MessageCatalogs = {}
+
+  for (const source of sources) {
+    if (!source) {
+      continue
+    }
+
+    for (const [locale, messages] of Object.entries(source)) {
+      catalogs[locale] = {
+        ...catalogs[locale],
+        ...messages,
+      }
+    }
+  }
+
+  return catalogs
+}

packages/i18n/src/runtime/context.ts

@@ -1,11 +1,11 @@
 import * as React from "react"
-import type { AllMessages } from "@lingui/core"
 
-import type { LocaleDefinition } from "./types"
+import type { IntlFormatters, LocaleDefinition, MessageCatalogs } from "./types"
 
 export interface I18nRuntimeContextValue {
-  catalogs: AllMessages
-  locale: string
+  catalogs: MessageCatalogs
+  formatters: IntlFormatters
+  locale: LocaleDefinition
   locales: readonly LocaleDefinition[]
 }
 

packages/i18n/src/runtime/hooks.ts

@@ -1,54 +1,87 @@
 import * as React from "react"
-import type { MessageDescriptor, MessageId, MessageOptions } from "@lingui/core"
 import { useLingui } from "@lingui/react"
 
 import { I18nRuntimeContext } from "./context"
-import type { LocaleDefinition, TranslateFunction } from "./types"
+import { createIntlFormatters } from "./intl-formatters"
+import type {
+  IntlFormatters,
+  LocaleDefinition,
+  MessageDescriptor,
+  MessageId,
+  MessageOptions,
+  MessageValues,
+  TranslateFunction,
+} from "./types"
 
 export function useTranslate(): TranslateFunction {
-  return useLingui()._
+  const { i18n } = useLingui()
+
+  return React.useCallback(
+    (descriptor, values) =>
+      i18n._({
+        ...descriptor,
+        values: values ?? descriptor.values,
+      }),
+    [i18n]
+  )
 }
 
 export function useMessage(descriptor: MessageDescriptor): string
 export function useMessage(
   id: MessageId,
-  values?: Record<string, unknown>,
+  values?: MessageValues,
   options?: MessageOptions
 ): string
 export function useMessage(
   descriptorOrId: MessageDescriptor | MessageId,
-  values?: Record<string, unknown>,
+  values?: MessageValues,
   options?: MessageOptions
 ) {
-  const translate = useTranslate()
+  const { i18n } = useLingui()
 
   return React.useMemo(
     () =>
       typeof descriptorOrId === "string"
-        ? translate(descriptorOrId, values, options)
-        : translate(descriptorOrId),
-    [descriptorOrId, options, translate, values]
+        ? i18n._(descriptorOrId, values, options)
+        : i18n._({
+            ...descriptorOrId,
+            values: values ?? descriptorOrId.values,
+          }),
+    [descriptorOrId, i18n, options, values]
   )
 }
 
 export function useLocale(): string {
-  const runtime = React.useContext(I18nRuntimeContext)
-  const { i18n } = useLingui()
-
-  return runtime?.locale ?? i18n.locale
+  return useLocaleDefinition().locale
 }
 
-export function useLocales(): readonly LocaleDefinition[] {
+export function useLocaleDefinition(): LocaleDefinition {
   const runtime = React.useContext(I18nRuntimeContext)
   const { i18n } = useLingui()
 
   return (
-    runtime?.locales ?? [
-      {
-        direction: "ltr",
-        label: i18n.locale,
-        locale: i18n.locale,
-      },
-    ]
+    runtime?.locale ?? {
+      label: i18n.locale,
+      languageTag: i18n.locale,
+      locale: i18n.locale,
+    }
   )
 }
+
+export function useLocales(): readonly LocaleDefinition[] {
+  const runtime = React.useContext(I18nRuntimeContext)
+  const activeLocale = useLocaleDefinition()
+
+  return runtime?.locales ?? [activeLocale]
+}
+
+export function useFormatters(): IntlFormatters {
+  const runtime = React.useContext(I18nRuntimeContext)
+  const locale = useLocaleDefinition()
+  const fallbackFormatters = React.useMemo(
+    () => createIntlFormatters(locale.languageTag ?? locale.locale),
+    [locale.languageTag, locale.locale]
+  )
+
+  return runtime?.formatters ?? fallbackFormatters
+}

packages/i18n/src/runtime/i18n-provider.tsx

@@ -1,12 +1,20 @@
 import * as React from "react"
-import { setupI18n, type AllMessages, type I18n } from "@lingui/core"
 import {
-  I18nProvider as LinguiI18nProvider,
-  type I18nProviderProps as LinguiI18nProviderProps,
-} from "@lingui/react"
+  setupI18n,
+  type AllMessages as LinguiMessageCatalogs,
+} from "@lingui/core"
+import { I18nProvider as LinguiI18nProvider } from "@lingui/react"
 
 import { I18nRuntimeContext } from "./context"
-import type { LocaleDefinition, LocaleDirection, LocaleInput } from "./types"
+import { mergeMessageCatalogs, type MessageCatalogSource } from "./catalogs"
+import { createIntlFormatters } from "./intl-formatters"
+import type {
+  LocaleDefinition,
+  LocaleDirection,
+  LocaleInput,
+  MessageCatalogs,
+  MissingTranslationHandler,
+} from "./types"
 
 const RTL_LANGUAGES = new Set([
   "ar",
@@ -28,10 +36,22 @@ function inferLocaleDirection(locale: string): LocaleDirection {
   return language && RTL_LANGUAGES.has(language) ? "rtl" : "ltr"
 }
 
+function getLocaleDisplayName(languageTag: string, fallback: string) {
+  try {
+    return (
+      new Intl.DisplayNames([languageTag], { type: "language" }).of(
+        languageTag
+      ) ?? fallback
+    )
+  } catch {
+    return fallback
+  }
+}
+
 function normalizeLocales(
   locale: string,
   locales: readonly LocaleInput[] | undefined,
-  catalogs: AllMessages
+  catalogs: MessageCatalogs
 ): readonly LocaleDefinition[] {
   const inputs =
     locales && locales.length > 0
@@ -40,12 +60,15 @@ function normalizeLocales(
 
   const definitions = inputs.map<LocaleDefinition>((input) => {
     const definition = typeof input === "string" ? { locale: input } : input
+    const languageTag = definition.languageTag ?? definition.locale
 
     return {
       ...definition,
-      direction:
-        definition.direction ?? inferLocaleDirection(definition.locale),
-      label: definition.label ?? definition.locale,
+      direction: definition.direction ?? inferLocaleDirection(languageTag),
+      label:
+        definition.label ??
+        getLocaleDisplayName(languageTag, definition.locale),
+      languageTag,
     }
   })
 
@@ -54,7 +77,8 @@ function normalizeLocales(
       ...definitions,
       {
         direction: inferLocaleDirection(locale),
-        label: locale,
+        label: getLocaleDisplayName(locale, locale),
+        languageTag: locale,
         locale,
       },
     ]
@@ -63,60 +87,80 @@ function normalizeLocales(
   return definitions
 }
 
-export interface I18nProviderProps extends Pick<
-  LinguiI18nProviderProps,
-  "children" | "defaultComponent"
-> {
-  catalogs?: AllMessages
-  i18n?: I18n
+export interface I18nProviderProps {
+  catalogs?: MessageCatalogSource | readonly MessageCatalogSource[]
+  children?: React.ReactNode
   locale: string
   locales?: readonly LocaleInput[]
-  missing?: ConstructorParameters<typeof I18n>[0]["missing"]
+  missing?: MissingTranslationHandler
+}
+
+function normalizeCatalogSources(
+  catalogs: I18nProviderProps["catalogs"]
+): readonly MessageCatalogSource[] {
+  if (!catalogs) {
+    return []
+  }
+
+  return Array.isArray(catalogs) ? catalogs : [catalogs as MessageCatalogSource]
 }
 
 export function I18nProvider({
-  catalogs = {},
+  catalogs,
   children,
-  defaultComponent,
-  i18n: providedI18n,
   locale,
   locales,
   missing,
 }: I18nProviderProps) {
-  const i18n = React.useMemo(() => {
-    const instance =
-      providedI18n ??
+  const mergedCatalogs = React.useMemo(
+    () => mergeMessageCatalogs(...normalizeCatalogSources(catalogs)),
+    [catalogs]
+  )
+  const normalizedLocales = React.useMemo(
+    () => normalizeLocales(locale, locales, mergedCatalogs),
+    [locale, locales, mergedCatalogs]
+  )
+  const activeLocale = React.useMemo(
+    () =>
+      normalizedLocales.find((definition) => definition.locale === locale) ?? {
+        direction: inferLocaleDirection(locale),
+        label: getLocaleDisplayName(locale, locale),
+        languageTag: locale,
+        locale,
+      },
+    [locale, normalizedLocales]
+  )
+  const formattingLocales = React.useMemo(
+    () => [activeLocale.languageTag ?? activeLocale.locale],
+    [activeLocale]
+  )
+  const i18n = React.useMemo(
+    () =>
       setupI18n({
         locale,
-        messages: catalogs,
+        locales: formattingLocales,
+        messages: mergedCatalogs as LinguiMessageCatalogs,
         missing,
-      })
-
-    if (providedI18n) {
-      providedI18n.load(catalogs)
-      providedI18n.activate(locale)
-    }
-
-    return instance
-  }, [catalogs, locale, missing, providedI18n])
-  const normalizedLocales = React.useMemo(
-    () => normalizeLocales(locale, locales, catalogs),
-    [catalogs, locale, locales]
+      }),
+    [formattingLocales, locale, mergedCatalogs, missing]
+  )
+  const formatters = React.useMemo(
+    () => createIntlFormatters(formattingLocales),
+    [formattingLocales]
   )
   const contextValue = React.useMemo(
     () => ({
-      catalogs,
-      locale,
+      catalogs: mergedCatalogs,
+      formatters,
+      locale: activeLocale,
       locales: normalizedLocales,
     }),
-    [catalogs, locale, normalizedLocales]
+    [activeLocale, formatters, mergedCatalogs, normalizedLocales]
   )
 
   return (
     <I18nRuntimeContext.Provider value={contextValue}>
-      <LinguiI18nProvider i18n={i18n} defaultComponent={defaultComponent}>
-        {children}
-      </LinguiI18nProvider>
+      <LinguiI18nProvider i18n={i18n}>{children}</LinguiI18nProvider>
     </I18nRuntimeContext.Provider>
   )
 }

packages/i18n/src/runtime/index.ts

@@ -1,15 +1,26 @@
 export { I18nProvider, type I18nProviderProps } from "./i18n-provider"
 export { Translate, type TranslateProps } from "./translate"
-export { useLocale, useLocales, useMessage, useTranslate } from "./hooks"
+export { createIntlFormatters } from "./intl-formatters"
+export { mergeMessageCatalogs, type MessageCatalogSource } from "./catalogs"
+export {
+  useFormatters,
+  useLocale,
+  useLocaleDefinition,
+  useLocales,
+  useMessage,
+  useTranslate,
+} from "./hooks"
 export type {
-  AllMessages,
-  I18n,
+  IntlFormatters,
   LocaleDefinition,
   LocaleDirection,
   LocaleInput,
+  MessageCatalog,
+  MessageCatalogs,
   MessageDescriptor,
   MessageId,
   MessageOptions,
-  Messages,
+  MessageValues,
+  MissingTranslationHandler,
   TranslateFunction,
 } from "./types"

packages/i18n/src/runtime/intl-formatters.ts

@@ -0,0 +1,58 @@
+import type { IntlFormatters } from "./types"
+
+function createFormatterCache<TOptions extends object, TFormatter>(
+  createFormatter: (options: TOptions) => TFormatter
+) {
+  const formatters = new Map<string, TFormatter>()
+
+  return (options: TOptions = {} as TOptions) => {
+    const key = JSON.stringify(options)
+    const cachedFormatter = formatters.get(key)
+
+    if (cachedFormatter) {
+      return cachedFormatter
+    }
+
+    const formatter = createFormatter(options)
+    formatters.set(key, formatter)
+
+    return formatter
+  }
+}
+
+export function createIntlFormatters(
+  locale: string | readonly string[]
+): IntlFormatters {
+  const locales = typeof locale === "string" ? locale : [...locale]
+  const getCollator = createFormatterCache(
+    (options) => new Intl.Collator(locales, options)
+  )
+  const getDateTimeFormatter = createFormatterCache(
+    (options) => new Intl.DateTimeFormat(locales, options)
+  )
+  const getListFormatter = createFormatterCache(
+    (options) => new Intl.ListFormat(locales, options)
+  )
+  const getNumberFormatter = createFormatterCache(
+    (options) => new Intl.NumberFormat(locales, options)
+  )
+  const getRelativeTimeFormatter = createFormatterCache(
+    (options) => new Intl.RelativeTimeFormat(locales, options)
+  )
+
+  return {
+    compare: (left, right, options) =>
+      getCollator(options).compare(left, right),
+    formatCurrency: (value, currency, options) =>
+      getNumberFormatter({
+        ...options,
+        currency,
+        style: "currency",
+      }).format(value),
+    formatDate: (value, options) => getDateTimeFormatter(options).format(value),
+    formatList: (values, options) => getListFormatter(options).format(values),
+    formatNumber: (value, options) => getNumberFormatter(options).format(value),
+    formatRelativeTime: (value, unit, options) =>
+      getRelativeTimeFormatter(options).format(value, unit),
+  }
+}

packages/i18n/src/runtime/runtime.test.tsx

@@ -5,7 +5,14 @@ import { render, screen } from "@testing-library/react"
 import { describe, expect, it } from "vitest"
 
 import { I18nProvider } from "./i18n-provider"
-import { useLocale, useLocales, useMessage, useTranslate } from "./hooks"
+import {
+  useFormatters,
+  useLocale,
+  useLocaleDefinition,
+  useLocales,
+  useMessage,
+  useTranslate,
+} from "./hooks"
 import { Translate } from "./translate"
 
 const catalogs = {
@@ -26,7 +33,7 @@ function HookProbe() {
   return (
     <output>
       {locale}|{locales.map((item) => item.locale).join(",")}|{message}|
-      {translate("greeting")}
+      {translate({ id: "greeting", message: "Greeting" })}
     </output>
   )
 }
@@ -49,6 +56,34 @@ describe("i18n runtime", () => {
     expect(screen.getByText("zh-Hans|en,zh-Hans|你好|你好")).toBeTruthy()
   })
 
+  it("merges package catalogs and lets application catalogs override them", () => {
+    render(
+      <I18nProvider
+        catalogs={[
+          {
+            "zh-Hans": {
+              greeting: "包内翻译",
+              packageOnly: "来自组件包",
+            },
+          },
+          {
+            "zh-Hans": {
+              greeting: "应用翻译",
+            },
+          },
+        ]}
+        locale="zh-Hans"
+      >
+        <output>
+          <Translate id="greeting" message="Greeting" />|
+          <Translate id="packageOnly" message="Package only" />
+        </output>
+      </I18nProvider>
+    )
+
+    expect(screen.getByText("应用翻译|来自组件包")).toBeTruthy()
+  })
+
   it("wraps Lingui Trans without adding a DOM element", () => {
     const { container } = render(
       <I18nProvider catalogs={catalogs} locale="en">
@@ -74,4 +109,75 @@ describe("i18n runtime", () => {
 
     expect(screen.getByText("rtl")).toBeTruthy()
   })
+
+  it("uses localized language names when locale labels are omitted", () => {
+    function LocaleLabelsProbe() {
+      return (
+        <span>
+          {useLocales()
+            .map((locale) => locale.label)
+            .join("|")}
+        </span>
+      )
+    }
+
+    render(
+      <I18nProvider locale="en" locales={["en", "zh-Hans"]}>
+        <LocaleLabelsProbe />
+      </I18nProvider>
+    )
+
+    expect(screen.getByText("English|简体中文")).toBeTruthy()
+  })
+
+  it("keeps catalog locales separate from Intl language tags", () => {
+    function LocaleProbe() {
+      const locale = useLocaleDefinition()
+
+      return (
+        <span>
+          {locale.locale}|{locale.languageTag}
+        </span>
+      )
+    }
+
+    render(
+      <I18nProvider
+        locale="zh-Hans"
+        locales={[{ languageTag: "zh-CN", locale: "zh-Hans" }]}
+      >
+        <LocaleProbe />
+      </I18nProvider>
+    )
+
+    expect(screen.getByText("zh-Hans|zh-CN")).toBeTruthy()
+  })
+
+  it("formats standard internationalized values through Intl", () => {
+    function FormatterProbe() {
+      const formatters = useFormatters()
+
+      return (
+        <output>
+          {formatters.formatNumber(1234.5)}|
+          {formatters.formatCurrency(1234.5, "USD")}|
+          {formatters.formatList(["A", "B", "C"])}|
+          {formatters.formatRelativeTime(-1, "day", { numeric: "auto" })}
+        </output>
+      )
+    }
+
+    render(
+      <I18nProvider
+        locale="en"
+        locales={[{ languageTag: "en-US", locale: "en" }]}
+      >
+        <FormatterProbe />
+      </I18nProvider>
+    )
+
+    expect(
+      screen.getByText("1,234.5|$1,234.50|A, B, and C|yesterday")
+    ).toBeTruthy()
+  })
 })

packages/i18n/src/runtime/translate.tsx

@@ -1,7 +1,13 @@
 import { Trans, type TransProps } from "@lingui/react"
 
-export type TranslateProps = TransProps
+import type { MessageDescriptor } from "./types"
+
+export type TranslateProps = Omit<
+  TransProps,
+  "comment" | "id" | "message" | "values"
+> &
+  MessageDescriptor
 
 export function Translate(props: TranslateProps) {
-  return <Trans {...props} />
+  return <Trans {...(props as TransProps)} />
 }

packages/i18n/src/runtime/types.ts

@@ -1,29 +1,77 @@
-import type {
-  AllMessages,
-  I18n,
-  MessageDescriptor,
-  MessageId,
-  MessageOptions,
-  Messages,
-} from "@lingui/core"
+export type MessageId = string
+export type MessageValues = Record<string, unknown>
 
-export type {
-  AllMessages,
-  I18n,
-  MessageDescriptor,
-  MessageId,
-  MessageOptions,
-  Messages,
+export interface MessageDescriptor {
+  id: MessageId
+  message: string
+  comment?: string
+  values?: MessageValues
 }
 
+export interface MessageOptions {
+  comment?: string
+  formats?: Record<
+    string,
+    Intl.DateTimeFormatOptions | Intl.NumberFormatOptions
+  >
+  message?: string
+}
+
+export type MessageCatalog = Record<string, unknown>
+export type MessageCatalogs = Record<string, MessageCatalog>
+
+export type MissingTranslationHandler =
+  string | ((locale: string, id: string) => string)
+
 export type LocaleDirection = "ltr" | "rtl"
 
 export interface LocaleDefinition {
   direction?: LocaleDirection
   label?: string
+
+  /**
+   * The message catalog identifier, for example `zh-Hans`.
+   */
   locale: string
+
+  /**
+   * The BCP-47 language tag used by Intl formatters, for example `zh-CN`.
+   * Defaults to `locale`.
+   */
+  languageTag?: string
 }
 
 export type LocaleInput = LocaleDefinition | string
 
-export type TranslateFunction = I18n["_"]
+export type TranslateFunction = (
+  descriptor: MessageDescriptor,
+  values?: MessageValues
+) => string
+
+export interface IntlFormatters {
+  compare(left: string, right: string, options?: Intl.CollatorOptions): number
+
+  formatCurrency(
+    value: number | bigint,
+    currency: string,
+    options?: Omit<Intl.NumberFormatOptions, "currency" | "style">
+  ): string
+
+  formatDate(value: Date | number, options?: Intl.DateTimeFormatOptions): string
+
+  formatList(
+    values: readonly string[],
+    options?: Intl.ListFormatOptions
+  ): string
+
+  formatNumber(
+    value: number | bigint,
+    options?: Intl.NumberFormatOptions
+  ): string
+
+  formatRelativeTime(
+    value: number,
+    unit: Intl.RelativeTimeFormatUnit,
+    options?: Intl.RelativeTimeFormatOptions
+  ): string
+}

packages/i18n/src/vite.ts

@@ -1,32 +1,273 @@
-import type { Plugin } from "vite"
+import { mkdir, readFile, writeFile } from "node:fs/promises"
+import { dirname, join } from "node:path"
+import type { Plugin, ResolvedConfig } from "vite"
 
+import {
+  loadProjectCatalogs,
+  resolveCatalogSourceFilenames,
+} from "./build/catalog-sources.ts"
+import {
+  compactMessageCatalogs,
+  createMessageIdSchema,
+  type MessageIdSchema,
+} from "./build/production-catalogs.ts"
 import { configureI18nApi } from "./cli/api-plugin.ts"
-import { resolveProjectFilename } from "./cli/project.ts"
+import {
+  getProjectRoot,
+  readProject,
+  resolveProjectFilename,
+} from "./cli/project.ts"
+import type { I18nProjectConfig } from "./config/lingui-config.ts"
+import type { MessageCatalogs } from "./runtime/types.ts"
+
+const CATALOG_MODULE_ID = "@workspace/i18n/catalogs"
+const RESOLVED_CATALOG_MODULE_ID = "\0workspace-i18n:catalogs"
+const REACT_EXTERNAL_STORE_SHIM_ID = "use-sync-external-store/shim"
+const RESOLVED_REACT_EXTERNAL_STORE_SHIM_ID =
+  "\0workspace-i18n:react-external-store-shim"
 
 export interface I18nPluginOptions {
   project?: string
 }
 
+async function writeFileIfChanged(filename: string, content: string) {
+  let currentContent: string | undefined
+
+  try {
+    currentContent = await readFile(filename, "utf8")
+  } catch {
+    // The generated file does not exist yet.
+  }
+
+  if (currentContent === content) {
+    return
+  }
+
+  await mkdir(dirname(filename), { recursive: true })
+  await writeFile(filename, content, "utf8")
+}
+
+function createCatalogLoaderRuntime(resolveCatalog: string) {
+  return `
+const catalogPromises = new Map()
+
+export function loadMessageCatalog(locale) {
+  let promise = catalogPromises.get(locale)
+
+  if (!promise) {
+    promise = Promise.resolve().then(() => ${resolveCatalog})
+    catalogPromises.set(locale, promise)
+  }
+
+  return promise
+}
+`
+}
+
+function createDevelopmentCatalogModule(
+  sourcesByLocale: Readonly<Record<string, readonly string[]>>
+) {
+  const imports: string[] = []
+  const catalogs: string[] = []
+  let importIndex = 0
+
+  for (const [locale, filenames] of Object.entries(sourcesByLocale)) {
+    const identifiers = filenames.map((filename) => {
+      const identifier = `catalog${importIndex}`
+
+      importIndex += 1
+      imports.push(
+        `import { messages as ${identifier} } from ${JSON.stringify(filename)}`
+      )
+
+      return identifier
+    })
+
+    catalogs.push(
+      `${JSON.stringify(locale)}: Object.assign({}, ${identifiers.join(", ")})`
+    )
+  }
+
+  return `${imports.join("\n")}
+
+const catalogs = { ${catalogs.join(", ")} }
+${createCatalogLoaderRuntime(
+  "catalogs[locale] ?? Promise.reject(new Error(`Unknown i18n locale: ${locale}`))"
+)}
+`
+}
+
+function createServerCatalogModule(catalogs: MessageCatalogs) {
+  return `const catalogs = ${JSON.stringify(catalogs)}
+${createCatalogLoaderRuntime(
+  "catalogs[locale] ?? Promise.reject(new Error(`Unknown i18n locale: ${locale}`))"
+)}
+`
+}
+
+function createClientCatalogModule(
+  assetReferences: Readonly<Record<string, string>>
+) {
+  const urls = Object.entries(assetReferences)
+    .map(
+      ([locale, reference]) =>
+        `${JSON.stringify(locale)}: import.meta.ROLLUP_FILE_URL_${reference}`
+    )
+    .join(", ")
+
+  return `const catalogUrls = { ${urls} }
+${createCatalogLoaderRuntime(`(async () => {
+      const url = catalogUrls[locale]
+
+      if (!url) {
+        throw new Error(\`Unknown i18n locale: \${locale}\`)
+      }
+
+      const response = await fetch(url)
+
+      if (!response.ok) {
+        throw new Error(
+          \`Unable to load i18n catalog for \${locale}: \${response.status}\`
+        )
+      }
+
+      return response.json()
+    })()`)}
+`
+}
+
+function replaceMessageIds(code: string, schema: MessageIdSchema) {
+  let transformedCode = code
+
+  for (const [id, compactId] of Object.entries(schema.messages).sort(
+    ([left], [right]) => right.length - left.length
+  )) {
+    transformedCode = transformedCode.replaceAll(
+      JSON.stringify(id),
+      JSON.stringify(compactId)
+    )
+    transformedCode = transformedCode.replaceAll(
+      `'${id.replaceAll("\\", "\\\\").replaceAll("'", "\\'")}'`,
+      `'${compactId}'`
+    )
+  }
+
+  return transformedCode
+}
+
 export function i18n(options: I18nPluginOptions = {}): Plugin {
-  let projectFilename: string | undefined
+  let config: ResolvedConfig
+  let project: I18nProjectConfig
+  let projectFilename: string
+  let projectRoot: string
+  let compactCatalogs: MessageCatalogs | undefined
+  let schema: MessageIdSchema | undefined
+  let sourcesByLocale: Record<string, readonly string[]> | undefined
+  let assetReferences: Record<string, string> = {}
 
   return {
     name: "workspace-i18n",
-    apply: "serve",
-    async configResolved(config) {
+    enforce: "pre",
+    async configResolved(resolvedConfig) {
+      config = resolvedConfig
       projectFilename = await resolveProjectFilename(
         options.project,
         config.root
       )
+      projectRoot = getProjectRoot(projectFilename)
+      project = await readProject(projectFilename)
     },
     configureServer(server) {
-      if (!projectFilename) {
-        throw new Error(
-          "The @workspace/i18n Vite plugin could not resolve i18n.config.json."
-        )
+      configureI18nApi(server, projectFilename)
+    },
+    resolveId(id) {
+      if (id === CATALOG_MODULE_ID) {
+        return RESOLVED_CATALOG_MODULE_ID
       }
 
-      configureI18nApi(server, projectFilename)
+      // Lingui supports older React versions through the external-store shim.
+      // This workspace requires React 19, so importing the built-in hook keeps
+      // Nitro SSR on the same React instance as the renderer.
+      if (id === REACT_EXTERNAL_STORE_SHIM_ID) {
+        return RESOLVED_REACT_EXTERNAL_STORE_SHIM_ID
+      }
+
+      return undefined
+    },
+    async buildStart() {
+      if (config.command !== "build") {
+        return
+      }
+
+      const loadedCatalogs = await loadProjectCatalogs(project, projectRoot)
+
+      sourcesByLocale = loadedCatalogs.sourcesByLocale
+      schema = createMessageIdSchema(loadedCatalogs.catalogs)
+      compactCatalogs = compactMessageCatalogs(loadedCatalogs.catalogs, schema)
+      assetReferences = {}
+
+      for (const locale of project.locales) {
+        assetReferences[locale] = this.emitFile({
+          name: `i18n/${locale}.json`,
+          source: JSON.stringify(compactCatalogs[locale] ?? {}),
+          type: "asset",
+        })
+      }
+
+      await writeFileIfChanged(
+        join(projectRoot, ".i18n", "schema.json"),
+        `${JSON.stringify(schema, null, 2)}\n`
+      )
+    },
+    async load(id, loadOptions) {
+      if (id === RESOLVED_REACT_EXTERNAL_STORE_SHIM_ID) {
+        return 'export { useSyncExternalStore } from "react"'
+      }
+
+      if (id !== RESOLVED_CATALOG_MODULE_ID) {
+        return undefined
+      }
+
+      if (config.command === "build") {
+        if (!compactCatalogs) {
+          throw new Error(
+            "Production i18n catalogs were not prepared before loading the catalog module."
+          )
+        }
+
+        return loadOptions?.ssr
+          ? createServerCatalogModule(compactCatalogs)
+          : createClientCatalogModule(assetReferences)
+      }
+
+      if (!sourcesByLocale) {
+        const resolvedSourcesByLocale = Object.fromEntries(
+          await Promise.all(
+            project.locales.map(async (locale) => [
+              locale,
+              await resolveCatalogSourceFilenames(project, projectRoot, locale),
+            ])
+          )
+        )
+
+        sourcesByLocale = resolvedSourcesByLocale
+      }
+
+      return createDevelopmentCatalogModule(sourcesByLocale!)
+    },
+    renderChunk(code) {
+      if (config.command !== "build" || !schema) {
+        return undefined
+      }
+
+      const transformedCode = replaceMessageIds(code, schema)
+
+      return transformedCode === code
+        ? undefined
+        : {
+            code: transformedCode,
+            map: null,
+          }
     },
   }
 }