Add i18n with i18next, German and English, and a key-parity check

Translation resources live in packages/shared/i18n/<lang>/<ns>.json
(common, errors) and ship with de and en. The web app initializes
react-i18next with bundled resources (?lng= wins, then the browser
language); all shell components use useTranslation and the temporary
t() stub is gone. The api localizes its uniform error bodies via a
minimal i18next instance negotiated from Accept-Language. `pnpm
i18n:check` fails CI when any key is missing in any language, backed
by tested helpers in @dorfteich/shared.

Closes #5

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/api/package.json

@@ -15,14 +15,15 @@
   },
   "dependencies": {
     "@dorfteich/shared": "workspace:*",
-    "@prisma/client": "^6.3.0",
-    "prisma": "^6.3.0",
     "@nestjs/common": "^11.0.0",
     "@nestjs/core": "^11.0.0",
     "@nestjs/platform-express": "^11.0.0",
+    "@prisma/client": "^6.3.0",
+    "i18next": "^26.3.4",
     "nestjs-pino": "^4.3.0",
     "pino": "^9.6.0",
     "pino-http": "^10.4.0",
+    "prisma": "^6.3.0",
     "reflect-metadata": "^0.2.2",
     "rxjs": "^7.8.0"
   },

apps/api/src/common/api-exception.filter.ts

@@ -1,12 +1,14 @@
 import { ArgumentsHost, Catch, ExceptionFilter, HttpException, HttpStatus } from '@nestjs/common';
 import { apiError } from '@dorfteich/shared';
-import type { Response } from 'express';
+import type { Request, Response } from 'express';
 import { PinoLogger } from 'nestjs-pino';
 
+import { negotiateLanguage, translateErrorCode } from '../i18n/api-i18n';
+
 /**
- * Maps every thrown error to the uniform ApiErrorBody shape. HttpExceptions
+ * Maps every thrown error to the uniform ApiErrorBody shape, localized via
- * keep their status and get a stable `code`; everything else becomes an
+ * Accept-Language. HttpExceptions keep their status and get a stable
- * opaque 500 so internals never leak to clients.
+ * `code`; everything else becomes an opaque 500 so internals never leak.
  */
 @Catch()
 export class ApiExceptionFilter implements ExceptionFilter {
@@ -15,18 +17,29 @@ export class ApiExceptionFilter implements ExceptionFilter {
   }
 
   catch(exception: unknown, host: ArgumentsHost): void {
-    const response = host.switchToHttp().getResponse<Response>();
+    const http = host.switchToHttp();
+    const response = http.getResponse<Response>();
+    const language = negotiateLanguage(http.getRequest<Request>().headers['accept-language']);
 
     if (exception instanceof HttpException) {
       const status = exception.getStatus();
-      response.status(status).json(apiError(codeForStatus(status), exception.message));
+      const code = codeForStatus(status);
+      // Catalogued codes get the localized text; uncatalogued ones keep
+      // the (developer-provided, English) exception message as fallback.
+      const message = translateErrorCode(code, language) ?? exception.message;
+      response.status(status).json(apiError(code, message));
       return;
     }
 
     this.logger.error({ err: exception }, 'unhandled exception');
     response
       .status(HttpStatus.INTERNAL_SERVER_ERROR)
-      .json(apiError('internal_error', 'Internal server error'));
+      .json(
+        apiError(
+          'internal_error',
+          translateErrorCode('internal_error', language) ?? 'Internal server error',
+        ),
+      );
   }
 }
 

apps/api/src/i18n/api-i18n.ts

@@ -0,0 +1,39 @@
+import deErrors from '@dorfteich/shared/i18n/de/errors.json';
+import enErrors from '@dorfteich/shared/i18n/en/errors.json';
+import { createInstance, type i18n as I18n } from 'i18next';
+
+/**
+ * Minimal i18next instance for user-facing api texts (error messages,
+ * later e-mails). Kept separate from any request context — callers pass
+ * the language explicitly.
+ */
+export const apiI18n: I18n = createInstance();
+
+void apiI18n.init({
+  resources: {
+    en: { errors: enErrors },
+    de: { errors: deErrors },
+  },
+  fallbackLng: 'en',
+  supportedLngs: ['de', 'en'],
+  interpolation: { escapeValue: false },
+});
+
+/** Best supported language for an Accept-Language header value. */
+export function negotiateLanguage(acceptLanguage: string | undefined): 'de' | 'en' {
+  if (!acceptLanguage) return 'en';
+  // First matching language tag wins; quality factors are ignored on
+  // purpose — with two languages the added complexity buys nothing.
+  for (const part of acceptLanguage.split(',')) {
+    const tag = part.trim().toLowerCase();
+    if (tag.startsWith('de')) return 'de';
+    if (tag.startsWith('en')) return 'en';
+  }
+  return 'en';
+}
+
+/** Localized message for an error code, or undefined when uncatalogued. */
+export function translateErrorCode(code: string, language: 'de' | 'en'): string | undefined {
+  const key = `errors:${code}`;
+  return apiI18n.exists(key) ? apiI18n.t(key, { lng: language }) : undefined;
+}

apps/web/package.json

@@ -15,8 +15,11 @@
   "dependencies": {
     "@dorfteich/shared": "workspace:*",
     "@tanstack/react-query": "^5.66.0",
+    "i18next": "^26.3.4",
+    "i18next-browser-languagedetector": "^8.2.1",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
+    "react-i18next": "^17.0.8",
     "react-router-dom": "^7.1.0"
   },
   "devDependencies": {

apps/web/src/i18n/index.ts

@@ -0,0 +1,30 @@
+import deCommon from '@dorfteich/shared/i18n/de/common.json';
+import deErrors from '@dorfteich/shared/i18n/de/errors.json';
+import enCommon from '@dorfteich/shared/i18n/en/common.json';
+import enErrors from '@dorfteich/shared/i18n/en/errors.json';
+import i18n from 'i18next';
+import LanguageDetector from 'i18next-browser-languagedetector';
+import { initReactI18next } from 'react-i18next';
+
+/**
+ * Translation resources are bundled (no async loading): the whole catalog
+ * is small, and the offline-capable editor (ADR 0003) must not depend on
+ * fetching language files. Detection: ?lng=… wins, then the browser
+ * language; the user-profile setting (issue #17) will be layered on top.
+ */
+void i18n
+  .use(LanguageDetector)
+  .use(initReactI18next)
+  .init({
+    resources: {
+      en: { common: enCommon, errors: enErrors },
+      de: { common: deCommon, errors: deErrors },
+    },
+    defaultNS: 'common',
+    fallbackLng: 'en',
+    supportedLngs: ['de', 'en'],
+    interpolation: { escapeValue: false }, // React already escapes.
+    detection: { order: ['querystring', 'navigator'], lookupQuerystring: 'lng', caches: [] },
+  });
+
+export default i18n;

apps/web/src/i18n/t.ts

@@ -1,8 +0,0 @@
-/**
- * Temporary translation stub so no component hard-codes strings directly.
- * Issue #5 replaces this module with the real i18next setup; call sites
- * (`t('key', 'English fallback')`) already match the final signature.
- */
-export function t(_key: string, fallback: string): string {
-  return fallback;
-}

apps/web/src/layout/Sidebar.tsx

@@ -1,4 +1,4 @@
-import { t } from '../i18n/t';
+import { useTranslation } from 'react-i18next';
 
 interface SidebarProps {
   collapsed: boolean;
@@ -10,15 +10,14 @@ interface SidebarProps {
  * must not reflow the main content beyond reclaiming the width.
  */
 export function Sidebar({ collapsed }: SidebarProps): React.JSX.Element {
+  const { t } = useTranslation();
   return (
     <nav
       className={collapsed ? 'sidebar sidebar--collapsed' : 'sidebar'}
       aria-hidden={collapsed}
-      aria-label={t('layout.sidebar.label', 'Pages')}
+      aria-label={t('layout.sidebar.label')}
     >
-      <p className="sidebar__hint">
+      <p className="sidebar__hint">{t('layout.sidebar.placeholder')}</p>
-        {t('layout.sidebar.placeholder', 'Your ponds and pages will appear here.')}
-      </p>
     </nav>
   );
 }

apps/web/src/layout/TopBar.tsx

@@ -1,13 +1,13 @@
+import { useTranslation } from 'react-i18next';
 import { Link } from 'react-router-dom';
 
-import { t } from '../i18n/t';
-
 interface TopBarProps {
   sidebarCollapsed: boolean;
   onToggleSidebar: () => void;
 }
 
 export function TopBar({ sidebarCollapsed, onToggleSidebar }: TopBarProps): React.JSX.Element {
+  const { t } = useTranslation();
   return (
     <header className="topbar">
       <button
@@ -15,11 +15,7 @@ export function TopBar({ sidebarCollapsed, onToggleSidebar }: TopBarProps): Reac
         className="icon-button"
         onClick={onToggleSidebar}
         aria-expanded={!sidebarCollapsed}
-        aria-label={
+        aria-label={sidebarCollapsed ? t('layout.sidebar.expand') : t('layout.sidebar.collapse')}
-          sidebarCollapsed
-            ? t('layout.sidebar.expand', 'Show sidebar')
-            : t('layout.sidebar.collapse', 'Hide sidebar')
-        }
       >
         {/* Simple hamburger glyph; replaced by an icon set later. */}
         <span aria-hidden>☰</span>
@@ -29,7 +25,7 @@ export function TopBar({ sidebarCollapsed, onToggleSidebar }: TopBarProps): Reac
       </Link>
       <span className="topbar__spacer" />
       {/* User menu arrives with authentication (issue #16). */}
-      <span className="sidebar__hint">{t('layout.user.anonymous', 'Not signed in')}</span>
+      <span className="sidebar__hint">{t('layout.user.anonymous')}</span>
     </header>
   );
 }

apps/web/src/main.tsx

@@ -4,6 +4,7 @@ import { createRoot } from 'react-dom/client';
 import { BrowserRouter } from 'react-router-dom';
 
 import { App } from './App';
+import './i18n';
 import './styles/tokens.css';
 import './styles/base.css';
 

apps/web/src/pages/HomePage.tsx

@@ -1,32 +1,24 @@
 import { useQuery } from '@tanstack/react-query';
+import { useTranslation } from 'react-i18next';
 
-import { t } from '../i18n/t';
 import { fetchHealth } from '../lib/api';
 
 export function HomePage(): React.JSX.Element {
+  const { t } = useTranslation();
   const health = useQuery({ queryKey: ['healthz'], queryFn: fetchHealth, retry: 1 });
 
   return (
     <>
-      <h1>{t('home.title', 'Welcome to Dorfteich')}</h1>
+      <h1>{t('home.title')}</h1>
-      <p>
+      <p>{t('home.intro')}</p>
-        {t(
+      {health.isPending && <span className="status-pill">{t('home.api.checking')}</span>}
-          'home.intro',
-          'Dorfteich is an open-source wiki with real-time collaboration. This instance is being set up.',
-        )}
-      </p>
-      {health.isPending && (
-        <span className="status-pill">{t('home.api.checking', 'Checking API …')}</span>
-      )}
       {health.isSuccess && (
         <span className="status-pill status-pill--ok">
-          {t('home.api.ok', 'API reachable')} · {health.data.version}
+          {t('home.api.ok')} · {health.data.version}
         </span>
       )}
       {health.isError && (
-        <span className="status-pill status-pill--error">
+        <span className="status-pill status-pill--error">{t('home.api.error')}</span>
-          {t('home.api.error', 'API not reachable')}
-        </span>
       )}
     </>
   );

apps/web/src/pages/NotFoundPage.tsx

@@ -1,13 +1,13 @@
+import { useTranslation } from 'react-i18next';
 import { Link } from 'react-router-dom';
 
-import { t } from '../i18n/t';
-
 export function NotFoundPage(): React.JSX.Element {
+  const { t } = useTranslation();
   return (
     <>
-      <h1>{t('notFound.title', 'Page not found')}</h1>
+      <h1>{t('notFound.title')}</h1>
-      <p>{t('notFound.body', 'The address you opened does not exist.')}</p>
+      <p>{t('notFound.body')}</p>
-      <Link to="/">{t('notFound.home', 'Back to the start page')}</Link>
+      <Link to="/">{t('notFound.home')}</Link>
     </>
   );
 }

eslint.config.mjs

@@ -12,6 +12,13 @@ export default tseslint.config(
   js.configs.recommended,
   ...tseslint.configs.recommended,
   prettier,
+  {
+    // Plain-Node maintenance scripts (no TypeScript, no bundler).
+    files: ['scripts/**/*.mjs'],
+    languageOptions: {
+      globals: { console: 'readonly', process: 'readonly' },
+    },
+  },
   {
     rules: {
       // Unused values are usually bugs; underscore-prefix marks intentional ones.

packages/shared/i18n/de/common.json

@@ -0,0 +1,27 @@
+{
+  "layout": {
+    "sidebar": {
+      "expand": "Seitenleiste einblenden",
+      "collapse": "Seitenleiste ausblenden",
+      "label": "Seiten",
+      "placeholder": "Deine Teiche und Seiten erscheinen hier."
+    },
+    "user": {
+      "anonymous": "Nicht angemeldet"
+    }
+  },
+  "home": {
+    "title": "Willkommen im Dorfteich",
+    "intro": "Dorfteich ist ein Open-Source-Wiki mit Echtzeit-Zusammenarbeit. Diese Instanz wird gerade eingerichtet.",
+    "api": {
+      "checking": "Prüfe API …",
+      "ok": "API erreichbar",
+      "error": "API nicht erreichbar"
+    }
+  },
+  "notFound": {
+    "title": "Seite nicht gefunden",
+    "body": "Die aufgerufene Adresse existiert nicht.",
+    "home": "Zurück zur Startseite"
+  }
+}

packages/shared/i18n/de/errors.json

@@ -0,0 +1,11 @@
+{
+  "bad_request": "Die Anfrage ist ungültig.",
+  "unauthorized": "Bitte melde dich an, um fortzufahren.",
+  "forbidden": "Dir fehlt die Berechtigung für diese Aktion.",
+  "not_found": "Die angeforderte Ressource existiert nicht.",
+  "conflict": "Die Anfrage steht im Widerspruch zum aktuellen Zustand.",
+  "gone": "Diese Ressource ist nicht mehr verfügbar.",
+  "payload_too_large": "Die übermittelten Daten sind zu groß.",
+  "rate_limited": "Zu viele Anfragen — bitte versuche es später erneut.",
+  "internal_error": "Interner Serverfehler."
+}

packages/shared/i18n/en/common.json

@@ -0,0 +1,27 @@
+{
+  "layout": {
+    "sidebar": {
+      "expand": "Show sidebar",
+      "collapse": "Hide sidebar",
+      "label": "Pages",
+      "placeholder": "Your ponds and pages will appear here."
+    },
+    "user": {
+      "anonymous": "Not signed in"
+    }
+  },
+  "home": {
+    "title": "Welcome to Dorfteich",
+    "intro": "Dorfteich is an open-source wiki with real-time collaboration. This instance is being set up.",
+    "api": {
+      "checking": "Checking API …",
+      "ok": "API reachable",
+      "error": "API not reachable"
+    }
+  },
+  "notFound": {
+    "title": "Page not found",
+    "body": "The address you opened does not exist.",
+    "home": "Back to the start page"
+  }
+}

packages/shared/i18n/en/errors.json

@@ -0,0 +1,11 @@
+{
+  "bad_request": "The request is invalid.",
+  "unauthorized": "Please sign in to continue.",
+  "forbidden": "You do not have permission for this action.",
+  "not_found": "The requested resource does not exist.",
+  "conflict": "The request conflicts with the current state.",
+  "gone": "This resource is no longer available.",
+  "payload_too_large": "The submitted data is too large.",
+  "rate_limited": "Too many requests — please try again later.",
+  "internal_error": "Internal server error."
+}

packages/shared/package.json

@@ -12,10 +12,12 @@
       "types": "./dist/index.d.ts",
       "import": "./dist/index.mjs",
       "require": "./dist/index.js"
-    }
+    },
+    "./i18n/*": "./i18n/*"
   },
   "files": [
-    "dist"
+    "dist",
+    "i18n"
   ],
   "scripts": {
     "build": "tsup src/index.ts --format esm,cjs --dts --clean",

packages/shared/src/i18n-tools.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest';
+
+import { missingTranslationKeys, translationKeys } from './i18n-tools';
+
+describe('i18n tooling', () => {
+  it('flattens nested keys', () => {
+    expect(translationKeys({ a: { b: 'x', c: { d: 'y' } }, e: 'z' })).toEqual([
+      'a.b',
+      'a.c.d',
+      'e',
+    ]);
+  });
+
+  it('finds keys missing in one language', () => {
+    const en = { home: { title: 'Hi', body: 'Text' } };
+    const de = { home: { title: 'Hallo' } };
+    expect(missingTranslationKeys(en, de)).toEqual(['home.body']);
+    expect(missingTranslationKeys(de, en)).toEqual([]);
+  });
+});

packages/shared/src/i18n-tools.ts

@@ -0,0 +1,24 @@
+/**
+ * Tooling helpers for translation resources. Used by the repo-level
+ * `pnpm i18n:check` script and its tests: every key must exist in every
+ * language, in both directions.
+ */
+
+type TranslationTree = { [key: string]: string | TranslationTree };
+
+/** Flattens {a:{b:"x"}} to ["a.b"]. */
+export function translationKeys(tree: TranslationTree, prefix = ''): string[] {
+  return Object.entries(tree).flatMap(([key, value]) => {
+    const path = prefix ? `${prefix}.${key}` : key;
+    return typeof value === 'string' ? [path] : translationKeys(value, path);
+  });
+}
+
+/** Keys present in `reference` but missing in `candidate`. */
+export function missingTranslationKeys(
+  reference: TranslationTree,
+  candidate: TranslationTree,
+): string[] {
+  const have = new Set(translationKeys(candidate));
+  return translationKeys(reference).filter((key) => !have.has(key));
+}

packages/shared/src/index.ts

@@ -1,3 +1,4 @@
 export * from './api-error';
 export * from './env';
 export * from './health';
+export * from './i18n-tools';

pnpm-lock.yaml

@@ -44,6 +44,9 @@ importers:
       '@prisma/client':
         specifier: ^6.3.0
         version: 6.19.3(prisma@6.19.3(typescript@5.9.3))(typescript@5.9.3)
+      i18next:
+        specifier: ^26.3.4
+        version: 26.3.4(typescript@5.9.3)
       nestjs-pino:
         specifier: ^4.3.0
         version: 4.6.1(@nestjs/common@11.1.27(reflect-metadata@0.2.2)(rxjs@7.8.2))(pino-http@10.5.0)(pino@9.14.0)(rxjs@7.8.2)
@@ -108,12 +111,21 @@ importers:
       '@tanstack/react-query':
         specifier: ^5.66.0
         version: 5.101.2(react@19.2.7)
+      i18next:
+        specifier: ^26.3.4
+        version: 26.3.4(typescript@5.9.3)
+      i18next-browser-languagedetector:
+        specifier: ^8.2.1
+        version: 8.2.1
       react:
         specifier: ^19.0.0
         version: 19.2.7
       react-dom:
         specifier: ^19.0.0
         version: 19.2.7(react@19.2.7)
+      react-i18next:
+        specifier: ^17.0.8
+        version: 17.0.8(i18next@26.3.4(typescript@5.9.3))(react-dom@19.2.7(react@19.2.7))(react@19.2.7)(typescript@5.9.3)
       react-router-dom:
         specifier: ^7.1.0
         version: 7.18.1(react-dom@19.2.7(react@19.2.7))(react@19.2.7)
@@ -254,6 +266,10 @@ packages:
     peerDependencies:
       '@babel/core': ^7.0.0-0
 
+  '@babel/runtime@7.29.7':
+    resolution: {integrity: sha512-Nq8OhGWiZIZGV6hLHoyAKLLcJihP/xFeBMGJoUrxTX2psI8dCifzLhZISFb+VWS3wFMRDmCGw5R+dOySCqPLhw==}
+    engines: {node: '>=6.9.0'}
+
   '@babel/template@7.29.7':
     resolution: {integrity: sha512-puq+Gf35oI24FeN11LkoUQFqv9uwNeWpxXZi/Ji3rRIoKAzKnxRaZ+Gkj0vKS9ZCiTESfng1N9LyOyXvo+m+Gg==}
     engines: {node: '>=6.9.0'}
@@ -2264,10 +2280,24 @@ packages:
   help-me@5.0.0:
     resolution: {integrity: sha512-7xgomUX6ADmcYzFik0HzAxh/73YlKR9bmFzf51CZwR+b6YtzU2m0u49hQCqV6SvlqIqsaxovfwdvbnsw3b/zpg==}
 
+  html-parse-stringify@3.0.1:
+    resolution: {integrity: sha512-KknJ50kTInJ7qIScF3jeaFRpMpE8/lfiTdzf/twXyPBLAGrLRTmkz3AdTnKeh40X8k9L2fdYwEp/42WGXIRGcg==}
+
   http-errors@2.0.1:
     resolution: {integrity: sha512-4FbRdAX+bSdmo4AUFuS0WNiPz8NgFt+r8ThgNWmlrjQjt1Q7ZR9+zTlce2859x4KSXrwIsaeTqDoKQmtP8pLmQ==}
     engines: {node: '>= 0.8'}
 
+  i18next-browser-languagedetector@8.2.1:
+    resolution: {integrity: sha512-bZg8+4bdmaOiApD7N7BPT9W8MLZG+nPTOFlLiJiT8uzKXFjhxw4v2ierCXOwB5sFDMtuA5G4kgYZ0AznZxQ/cw==}
+
+  i18next@26.3.4:
+    resolution: {integrity: sha512-pa7m0d7pBDqGHZxljT+WPFeyFgQ7P7SciPPo1tTqYuO0z4sqADYhwnBESmmGp/wEof1inwdls/k8ZgTg8rxFHA==}
+    peerDependencies:
+      typescript: ^5 || ^6
+    peerDependenciesMeta:
+      typescript:
+        optional: true
+
   iconv-lite@0.7.2:
     resolution: {integrity: sha512-im9DjEDQ55s9fL4EYzOAv0yMqmMBSZp6G0VvFyTMPKWxiSBHUj9NW/qqLmXUwXrrM7AvqSlTCfvqRb0cM8yYqw==}
     engines: {node: '>=0.10.0'}
@@ -2774,6 +2804,22 @@ packages:
     peerDependencies:
       react: ^19.2.7
 
+  react-i18next@17.0.8:
+    resolution: {integrity: sha512-0ooKbGLU8JXhe1zwpQUWIeXSgLPOfwJmgheWRIUpcoA0CpyabpGhayjdG+/eA5esC1AQ8h2jWpXjJfzQzeDOCw==}
+    peerDependencies:
+      i18next: '>= 26.2.0'
+      react: '>= 16.8.0'
+      react-dom: '*'
+      react-native: '*'
+      typescript: ^5 || ^6
+    peerDependenciesMeta:
+      react-dom:
+        optional: true
+      react-native:
+        optional: true
+      typescript:
+        optional: true
+
   react-refresh@0.17.0:
     resolution: {integrity: sha512-z6F7K9bV85EfseRCp2bzrpyQ0Gkw1uLoCel9XBVWPg/TjRj94SkJzUTGfOa4bs7iJvBWtQG0Wq7wnI0syw3EBQ==}
     engines: {node: '>=0.10.0'}
@@ -3232,6 +3278,11 @@ packages:
   uri-js@4.4.1:
     resolution: {integrity: sha512-7rKUyy33Q1yc98pQ1DAmLtwX109F7TIfWlW1Ydo8Wl1ii1SeHieeh0HHfPeL2fMXK6z0s8ecKs9frCuLJvndBg==}
 
+  use-sync-external-store@1.6.0:
+    resolution: {integrity: sha512-Pp6GSwGP/NrPIrxVFAIkOQeyw8lFenOHijQWkUTrDvrF4ALqylP2C/KCkeS9dpUM3KvYRQhna5vt7IL95+ZQ9w==}
+    peerDependencies:
+      react: ^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0
+
   util-deprecate@1.0.2:
     resolution: {integrity: sha512-EPD5q1uXyFxJpCrLnCc1nHnq3gOa6DZBocAIiI2TaSCA7VCJ1UJDMagCzIkXNsUYfD1daK//LTEQ8xiIbrHtcw==}
 
@@ -3352,6 +3403,10 @@ packages:
       jsdom:
         optional: true
 
+  void-elements@3.1.0:
+    resolution: {integrity: sha512-Dhxzh5HZuiHQhbvTW9AMetFfBHDMYpo23Uo9btPXgdYP+3T5S+p+jgNy7spra+veYhBP2dCSgxR/i2Y02h5/6w==}
+    engines: {node: '>=0.10.0'}
+
   watchpack@2.5.2:
     resolution: {integrity: sha512-6i/00NBjP4yGPs+caKSyRfpTF/8Torsu0MOW3mMzIbhgISFder8i7xbqgHlLMwJrdiN8ndBV3UA1/AfzPSr+jg==}
     engines: {node: '>=10.13.0'}
@@ -3564,6 +3619,8 @@ snapshots:
       '@babel/core': 7.29.7
       '@babel/helper-plugin-utils': 7.29.7
 
+  '@babel/runtime@7.29.7': {}
+
   '@babel/template@7.29.7':
     dependencies:
       '@babel/code-frame': 7.29.7
@@ -5464,6 +5521,10 @@ snapshots:
 
   help-me@5.0.0: {}
 
+  html-parse-stringify@3.0.1:
+    dependencies:
+      void-elements: 3.1.0
+
   http-errors@2.0.1:
     dependencies:
       depd: 2.0.0
@@ -5472,6 +5533,14 @@ snapshots:
       statuses: 2.0.2
       toidentifier: 1.0.1
 
+  i18next-browser-languagedetector@8.2.1:
+    dependencies:
+      '@babel/runtime': 7.29.7
+
+  i18next@26.3.4(typescript@5.9.3):
+    optionalDependencies:
+      typescript: 5.9.3
+
   iconv-lite@0.7.2:
     dependencies:
       safer-buffer: 2.1.2
@@ -5919,6 +5988,17 @@ snapshots:
       react: 19.2.7
       scheduler: 0.27.0
 
+  react-i18next@17.0.8(i18next@26.3.4(typescript@5.9.3))(react-dom@19.2.7(react@19.2.7))(react@19.2.7)(typescript@5.9.3):
+    dependencies:
+      '@babel/runtime': 7.29.7
+      html-parse-stringify: 3.0.1
+      i18next: 26.3.4(typescript@5.9.3)
+      react: 19.2.7
+      use-sync-external-store: 1.6.0(react@19.2.7)
+    optionalDependencies:
+      react-dom: 19.2.7(react@19.2.7)
+      typescript: 5.9.3
+
   react-refresh@0.17.0: {}
 
   react-router-dom@7.18.1(react-dom@19.2.7(react@19.2.7))(react@19.2.7):
@@ -6386,6 +6466,10 @@ snapshots:
     dependencies:
       punycode: 2.3.1
 
+  use-sync-external-store@1.6.0(react@19.2.7):
+    dependencies:
+      react: 19.2.7
+
   util-deprecate@1.0.2: {}
 
   vary@1.1.2: {}
@@ -6482,6 +6566,8 @@ snapshots:
       - tsx
       - yaml
 
+  void-elements@3.1.0: {}
+
   watchpack@2.5.2:
     dependencies:
       graceful-fs: 4.2.11

scripts/i18n-check.mjs

@@ -0,0 +1,52 @@
+#!/usr/bin/env node
+/**
+ * Verifies that every translation key exists in every language, in both
+ * directions. Run via `pnpm i18n:check`; requires @dorfteich/shared to be
+ * built (`pnpm build`) because it imports the shared tooling helpers.
+ */
+import { readdirSync, readFileSync } from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+import { missingTranslationKeys } from '../packages/shared/dist/index.mjs';
+
+const i18nDir = path.join(path.dirname(fileURLToPath(import.meta.url)), '../packages/shared/i18n');
+const languages = readdirSync(i18nDir).sort();
+
+if (languages.length < 2) {
+  console.error(`i18n:check: expected at least two languages in ${i18nDir}`);
+  process.exit(1);
+}
+
+const namespaces = new Set(languages.flatMap((lang) => readdirSync(path.join(i18nDir, lang))));
+
+let problems = 0;
+for (const namespaceFile of [...namespaces].sort()) {
+  const trees = {};
+  for (const lang of languages) {
+    try {
+      trees[lang] = JSON.parse(readFileSync(path.join(i18nDir, lang, namespaceFile), 'utf8'));
+    } catch {
+      console.error(`✗ ${lang}/${namespaceFile}: missing or invalid JSON`);
+      problems += 1;
+      trees[lang] = {};
+    }
+  }
+  for (const reference of languages) {
+    for (const candidate of languages) {
+      if (reference === candidate) continue;
+      for (const key of missingTranslationKeys(trees[reference], trees[candidate])) {
+        console.error(
+          `✗ ${candidate}/${namespaceFile}: missing key "${key}" (present in ${reference})`,
+        );
+        problems += 1;
+      }
+    }
+  }
+}
+
+if (problems > 0) {
+  console.error(`i18n:check: ${problems} problem(s) found.`);
+  process.exit(1);
+}
+console.log(`i18n:check: ${languages.join(', ')} — all keys present in all languages.`);