diff --git a/src/main/claude/agent-runner-message-end.ts b/src/main/claude/agent-runner-message-end.ts
index 7929ab9b..112fe3b5 100644
--- a/src/main/claude/agent-runner-message-end.ts
+++ b/src/main/claude/agent-runner-message-end.ts
@@ -6,6 +6,7 @@ import type {
   ToolCall,
 } from '@mariozechner/pi-ai';
 import { splitThinkTagBlocks } from './think-tag-parser';
+import { mt } from '../i18n';
 
 type MessageEndContentBlock = TextContent | ThinkingContent | ToolCall;
 
@@ -43,31 +44,31 @@ export type AbortDisposition = 'timeout' | 'loop_guard' | 'stream_error' | 'user
 export function toUserFacingErrorText(errorText: string): string {
   const lower = errorText.toLowerCase();
   if (lower.includes('first_response_timeout')) {
-    return '模型响应超时：长时间未收到上游返回，请稍后重试或检查当前模型/网关负载。';
+    return mt('errModelTimeout');
   }
   if (lower.includes('empty_success_result')) {
-    return '模型返回了一个空的成功结果，当前模型或网关兼容性可能有问题，请重试或切换协议后再试。';
+    return mt('errEmptySuccess');
   }
   if (
     /\b400\b/.test(errorText) ||
     lower.includes('bad request') ||
     lower.includes('invalid request')
   ) {
-    return `请求被上游拒绝（400），可能是模型/协议配置不兼容。请检查模型名称、协议设置和 API 端点。\n原始错误: ${errorText}`;
+    return mt('errBadRequest', { error: errorText });
   }
   if (
     /\b(401|403)\b/.test(errorText) ||
     lower.includes('unauthorized') ||
     lower.includes('forbidden')
   ) {
-    return `认证失败，请检查 API Key 是否正确、是否已过期或无权访问当前模型。\n原始错误: ${errorText}`;
+    return mt('errAuthFailed', { error: errorText });
   }
   if (
     /\b429\b/.test(errorText) ||
     lower.includes('rate limit') ||
     lower.includes('too many requests')
   ) {
-    return `请求被限流（429），当前模型或 API 端点的调用频率已达上限，请稍后重试。\n原始错误: ${errorText}`;
+    return mt('errRateLimited', { error: errorText });
   }
   if (
     /\b(5\d{2})\b/.test(errorText) ||
@@ -76,7 +77,7 @@ export function toUserFacingErrorText(errorText: string): string {
     lower.includes('service unavailable') ||
     lower.includes('overloaded')
   ) {
-    return `上游服务异常，可能是模型服务过载或临时故障，SDK 将自动重试。\n原始错误: ${errorText}`;
+    return mt('errUpstreamError', { error: errorText });
   }
   if (
     lower.includes('terminated') ||
@@ -90,7 +91,7 @@ export function toUserFacingErrorText(errorText: string): string {
     lower.includes('upstream connect') ||
     lower.includes('retry delay')
   ) {
-    return `网络连接中断（${errorText}），可能是代理/网关不稳定，SDK 将自动重试。`;
+    return mt('errNetworkInterrupted', { error: errorText });
   }
   return errorText;
 }
@@ -104,9 +105,7 @@ export function resolveAssistantStreamErrorText(
 
 export function buildTerminalErrorMessage(errorText: string, partialText = ''): string {
   const normalizedPartial = partialText.trimEnd();
-  const hint = FOUR_XX_ERROR_RE.test(errorText)
-    ? '_请检查配置后重试。_'
-    : '_Agent 正在自动重试，请稍候..._';
+  const hint = FOUR_XX_ERROR_RE.test(errorText) ? mt('errCheckConfigHint') : mt('errRetryingHint');
   const errorBlock = `**Error**: ${errorText}\n\n${hint}`;
   return normalizedPartial ? `${normalizedPartial}\n\n${errorBlock}` : errorBlock;
 }
diff --git a/src/main/config/config-store.ts b/src/main/config/config-store.ts
index 136a53b0..48d5c2dd 100644
--- a/src/main/config/config-store.ts
+++ b/src/main/config/config-store.ts
@@ -30,6 +30,7 @@ import {
   shouldUseAnthropicAuthToken,
 } from './auth-utils';
 import { API_PROVIDER_PRESETS, PI_AI_CURATED_PRESETS } from '../../shared/api-model-presets';
+import { mt, setBackendLanguage, DEFAULT_BACKEND_LANGUAGE } from '../i18n';
 
 /**
  * Application configuration schema
@@ -112,6 +113,10 @@ export interface AppConfig {
   // UI theme preference
   theme: AppTheme;
 
+  // UI language. Mirrors the renderer's active react-i18next language and drives
+  // backend (main-process) strings via setBackendLanguage(). Defaults to Chinese.
+  uiLanguage?: string;
+
   // Sandbox mode (WSL/Lima isolation)
   sandboxEnabled: boolean;
 
@@ -218,7 +223,7 @@ const defaultProfiles: Record<ProviderProfileKey, ProviderProfile> = {
 
 const defaultConfigSet: ApiConfigSet = {
   id: DEFAULT_CONFIG_SET_ID,
-  name: '默认方案',
+  name: mt('configDefaultSetName'),
   isSystem: true,
   provider: 'openrouter',
   customProtocol: 'anthropic',
@@ -243,6 +248,7 @@ const defaultConfig: AppConfig = {
   globalSkillsPath: '',
   enableDevLogs: false,
   theme: 'light',
+  uiLanguage: DEFAULT_BACKEND_LANGUAGE,
   sandboxEnabled: false,
   memoryEnabled: true,
   memoryRuntime: {
@@ -539,6 +545,9 @@ export class ConfigStore {
       warn: logWarn,
     }) as unknown as Store<AppConfig>;
     this.ensureNormalized();
+    // Seed the backend translator from the persisted UI language so main-process
+    // strings (errors, dialogs) match the user's last choice from first launch.
+    setBackendLanguage((this.store.store as AppConfig).uiLanguage);
   }
 
   private ensureNormalized(): void {
@@ -858,7 +867,7 @@ export class ConfigStore {
 
       const normalizedSet = this.normalizeConfigSet(rawSet, {
         id: nextId,
-        name: toNonEmptyString(rawSet.name) || `方案 ${index + 1}`,
+        name: toNonEmptyString(rawSet.name) || mt('configFallbackSetName', { index: index + 1 }),
         provider: legacy.provider,
         customProtocol: legacy.customProtocol,
         activeProfileKey: legacy.activeProfileKey,
@@ -974,6 +983,10 @@ export class ConfigStore {
           : defaultConfig.globalSkillsPath,
       enableDevLogs: toBoolean(raw.enableDevLogs, defaultConfig.enableDevLogs),
       theme: isAppTheme(raw.theme) ? raw.theme : defaultConfig.theme,
+      uiLanguage:
+        typeof raw.uiLanguage === 'string' && raw.uiLanguage.trim()
+          ? raw.uiLanguage
+          : defaultConfig.uiLanguage,
       sandboxEnabled: toBoolean(raw.sandboxEnabled, defaultConfig.sandboxEnabled),
       memoryEnabled: toBoolean(raw.memoryEnabled, defaultConfig.memoryEnabled),
       memoryRuntime: normalizeMemoryRuntimeConfig(raw.memoryRuntime),
@@ -995,6 +1008,8 @@ export class ConfigStore {
   private saveConfig(config: AppConfig): void {
     const normalized = this.normalizeConfig(config);
     this.store.set(normalized);
+    // Keep the backend translator in sync when the UI language changes.
+    setBackendLanguage(normalized.uiLanguage);
   }
 
   private composeProjectedConfig(
@@ -1388,6 +1403,7 @@ export class ConfigStore {
       enableDevLogs:
         updates.enableDevLogs !== undefined ? updates.enableDevLogs : current.enableDevLogs,
       theme: updates.theme !== undefined ? updates.theme : current.theme,
+      uiLanguage: updates.uiLanguage !== undefined ? updates.uiLanguage : current.uiLanguage,
       sandboxEnabled:
         updates.sandboxEnabled !== undefined ? updates.sandboxEnabled : current.sandboxEnabled,
       memoryEnabled:
diff --git a/src/main/i18n/catalog.ts b/src/main/i18n/catalog.ts
new file mode 100644
index 00000000..b93639f9
--- /dev/null
+++ b/src/main/i18n/catalog.ts
@@ -0,0 +1,334 @@
+// Main-process (backend) localization catalog.
+//
+// Chinese (zh) is the source language and the default — the project owner ships
+// a Chinese-first product, so any key missing from another language falls back
+// to zh (see ./index.ts). The keys below cover the backend strings that surface
+// directly in the UI: model/network error messages, the "no credentials" prompt,
+// the startup-failure dialog, and the default config-set name.
+//
+// Placeholders use the {{name}} syntax and are interpolated by mt(); `\n`
+// produces a real newline at runtime, and leading/trailing underscores are
+// markdown italics rendered by the chat view.
+
+export type BackendMessageKey =
+  | 'errModelTimeout'
+  | 'errEmptySuccess'
+  | 'errBadRequest'
+  | 'errAuthFailed'
+  | 'errRateLimited'
+  | 'errUpstreamError'
+  | 'errNetworkInterrupted'
+  | 'errCheckConfigHint'
+  | 'errRetryingHint'
+  | 'errConfigRequired'
+  | 'startupFailedTitle'
+  | 'startupFailedBody'
+  | 'configDefaultSetName'
+  | 'configFallbackSetName';
+
+export type BackendMessages = Record<BackendMessageKey, string>;
+
+export const SUPPORTED_BACKEND_LANGUAGES = [
+  'en',
+  'zh',
+  'es',
+  'fr',
+  'de',
+  'it',
+  'uk',
+  'pl',
+  'sv',
+  'no',
+  'nl',
+  'ro',
+] as const;
+
+export const DEFAULT_BACKEND_LANGUAGE = 'zh';
+
+export const backendCatalog: Record<string, BackendMessages> = {
+  zh: {
+    errModelTimeout: '模型响应超时：长时间未收到上游返回，请稍后重试或检查当前模型/网关负载。',
+    errEmptySuccess:
+      '模型返回了一个空的成功结果，当前模型或网关兼容性可能有问题，请重试或切换协议后再试。',
+    errBadRequest:
+      '请求被上游拒绝（400），可能是模型/协议配置不兼容。请检查模型名称、协议设置和 API 端点。\n原始错误: {{error}}',
+    errAuthFailed:
+      '认证失败，请检查 API Key 是否正确、是否已过期或无权访问当前模型。\n原始错误: {{error}}',
+    errRateLimited:
+      '请求被限流（429），当前模型或 API 端点的调用频率已达上限，请稍后重试。\n原始错误: {{error}}',
+    errUpstreamError:
+      '上游服务异常，可能是模型服务过载或临时故障，SDK 将自动重试。\n原始错误: {{error}}',
+    errNetworkInterrupted: '网络连接中断（{{error}}），可能是代理/网关不稳定，SDK 将自动重试。',
+    errCheckConfigHint: '_请检查配置后重试。_',
+    errRetryingHint: '_Agent 正在自动重试，请稍候..._',
+    errConfigRequired: '当前方案未配置可用凭证，请先在 API 设置中完成配置',
+    startupFailedTitle: 'Open Cowork 启动失败',
+    startupFailedBody: '{{message}}\n\n请查看日志获取更多信息。',
+    configDefaultSetName: '默认方案',
+    configFallbackSetName: '方案 {{index}}',
+  },
+  en: {
+    errModelTimeout:
+      'Model response timed out: no reply from the upstream service for a while. Please retry later or check the current model/gateway load.',
+    errEmptySuccess:
+      'The model returned an empty successful result. The current model or gateway may have a compatibility issue — please retry or switch protocol and try again.',
+    errBadRequest:
+      'The request was rejected upstream (400). The model/protocol configuration may be incompatible. Please check the model name, protocol settings and API endpoint.\nOriginal error: {{error}}',
+    errAuthFailed:
+      'Authentication failed. Please check whether the API Key is correct, has expired, or lacks access to the current model.\nOriginal error: {{error}}',
+    errRateLimited:
+      'The request was rate limited (429). The current model or API endpoint has reached its call-rate limit. Please retry later.\nOriginal error: {{error}}',
+    errUpstreamError:
+      'The upstream service returned an error — the model service may be overloaded or temporarily unavailable. The SDK will retry automatically.\nOriginal error: {{error}}',
+    errNetworkInterrupted:
+      'The network connection was interrupted ({{error}}). The proxy/gateway may be unstable. The SDK will retry automatically.',
+    errCheckConfigHint: '_Please check your configuration and retry._',
+    errRetryingHint: '_The agent is retrying automatically, please wait..._',
+    errConfigRequired:
+      'The current configuration set has no usable credentials. Please complete the setup in API Settings first.',
+    startupFailedTitle: 'Open Cowork failed to start',
+    startupFailedBody: '{{message}}\n\nPlease check the logs for more information.',
+    configDefaultSetName: 'Default',
+    configFallbackSetName: 'Configuration {{index}}',
+  },
+  es: {
+    errModelTimeout:
+      'Se agotó el tiempo de espera de la respuesta del modelo: el servicio remoto no respondió durante un rato. Vuelve a intentarlo más tarde o revisa la carga actual del modelo o la pasarela.',
+    errEmptySuccess:
+      'El modelo devolvió un resultado correcto pero vacío. Es posible que el modelo o la pasarela actual tengan un problema de compatibilidad; vuelve a intentarlo o cambia de protocolo e inténtalo de nuevo.',
+    errBadRequest:
+      'El servicio remoto rechazó la solicitud (400). Puede que la configuración del modelo o el protocolo sea incompatible. Comprueba el nombre del modelo, los ajustes del protocolo y el endpoint de la API.\nError original: {{error}}',
+    errAuthFailed:
+      'Error de autenticación. Comprueba si la API Key es correcta, ha caducado o no tiene acceso al modelo actual.\nError original: {{error}}',
+    errRateLimited:
+      'Se limitó la frecuencia de la solicitud (429). El modelo o el endpoint de la API actual alcanzó su límite de frecuencia de llamadas. Vuelve a intentarlo más tarde.\nError original: {{error}}',
+    errUpstreamError:
+      'El servicio remoto devolvió un error: puede que el servicio del modelo esté sobrecargado o no esté disponible temporalmente. El SDK reintentará automáticamente.\nError original: {{error}}',
+    errNetworkInterrupted:
+      'Se interrumpió la conexión de red ({{error}}). Puede que el proxy o la pasarela sean inestables. El SDK reintentará automáticamente.',
+    errCheckConfigHint: '_Comprueba tu configuración y vuelve a intentarlo._',
+    errRetryingHint: '_El agente está reintentando automáticamente, espera un momento..._',
+    errConfigRequired:
+      'El conjunto de configuración actual no tiene credenciales utilizables. Completa primero la configuración en los Ajustes de la API.',
+    startupFailedTitle: 'No se pudo iniciar Open Cowork',
+    startupFailedBody: '{{message}}\n\nConsulta los registros para obtener más información.',
+    configDefaultSetName: 'Predeterminada',
+    configFallbackSetName: 'Configuración {{index}}',
+  },
+  fr: {
+    errModelTimeout:
+      "Le délai d'attente de la réponse du modèle a expiré : aucune réponse du service en amont depuis un certain temps. Veuillez réessayer plus tard ou vérifier la charge actuelle du modèle ou de la passerelle.",
+    errEmptySuccess:
+      'Le modèle a renvoyé un résultat vide alors que la requête a abouti. Le modèle ou la passerelle actuels présentent peut-être un problème de compatibilité — veuillez réessayer ou changer de protocole, puis recommencer.',
+    errBadRequest:
+      "La requête a été rejetée en amont (400). La configuration du modèle ou du protocole est peut-être incompatible. Veuillez vérifier le nom du modèle, les paramètres du protocole et le point de terminaison de l'API.\nErreur d'origine : {{error}}",
+    errAuthFailed:
+      "Échec de l'authentification. Veuillez vérifier si l'API Key est correcte, si elle a expiré ou si elle n'a pas accès au modèle actuel.\nErreur d'origine : {{error}}",
+    errRateLimited:
+      "La requête a été limitée en débit (429). Le modèle ou le point de terminaison de l'API actuels ont atteint leur limite de fréquence d'appels. Veuillez réessayer plus tard.\nErreur d'origine : {{error}}",
+    errUpstreamError:
+      "Le service en amont a renvoyé une erreur — le service du modèle est peut-être surchargé ou temporairement indisponible. Le SDK réessaiera automatiquement.\nErreur d'origine : {{error}}",
+    errNetworkInterrupted:
+      'La connexion réseau a été interrompue ({{error}}). Le proxy ou la passerelle sont peut-être instables. Le SDK réessaiera automatiquement.',
+    errCheckConfigHint: '_Veuillez vérifier votre configuration et réessayer._',
+    errRetryingHint: "_L'agent réessaie automatiquement, veuillez patienter..._",
+    errConfigRequired:
+      "Le jeu de configuration actuel ne contient aucun identifiant utilisable. Veuillez d'abord finaliser la configuration dans les Paramètres de l'API.",
+    startupFailedTitle: "Échec du démarrage d'Open Cowork",
+    startupFailedBody: '{{message}}\n\nVeuillez consulter les journaux pour plus d’informations.',
+    configDefaultSetName: 'Par défaut',
+    configFallbackSetName: 'Configuration {{index}}',
+  },
+  de: {
+    errModelTimeout:
+      'Zeitüberschreitung bei der Modellantwort: Der vorgelagerte Dienst hat längere Zeit nicht reagiert. Bitte versuchen Sie es später erneut oder prüfen Sie die aktuelle Auslastung von Modell/Gateway.',
+    errEmptySuccess:
+      'Das Modell hat ein leeres, aber erfolgreiches Ergebnis zurückgegeben. Möglicherweise besteht ein Kompatibilitätsproblem mit dem aktuellen Modell oder Gateway – bitte versuchen Sie es erneut oder wechseln Sie das Protokoll.',
+    errBadRequest:
+      'Die Anfrage wurde vorgelagert abgelehnt (400). Die Konfiguration von Modell/Protokoll ist möglicherweise inkompatibel. Bitte prüfen Sie Modellname, Protokolleinstellungen und API-Endpunkt.\nUrsprünglicher Fehler: {{error}}',
+    errAuthFailed:
+      'Authentifizierung fehlgeschlagen. Bitte prüfen Sie, ob der API Key korrekt ist, abgelaufen ist oder keinen Zugriff auf das aktuelle Modell hat.\nUrsprünglicher Fehler: {{error}}',
+    errRateLimited:
+      'Die Anfrage wurde wegen einer Ratenbegrenzung abgelehnt (429). Das aktuelle Modell oder der API-Endpunkt hat sein Aufruflimit erreicht. Bitte versuchen Sie es später erneut.\nUrsprünglicher Fehler: {{error}}',
+    errUpstreamError:
+      'Der vorgelagerte Dienst hat einen Fehler zurückgegeben – der Modelldienst ist möglicherweise überlastet oder vorübergehend nicht verfügbar. Das SDK wiederholt den Vorgang automatisch.\nUrsprünglicher Fehler: {{error}}',
+    errNetworkInterrupted:
+      'Die Netzwerkverbindung wurde unterbrochen ({{error}}). Möglicherweise ist der Proxy bzw. das Gateway instabil. Das SDK wiederholt den Vorgang automatisch.',
+    errCheckConfigHint: '_Bitte überprüfen Sie Ihre Konfiguration und versuchen Sie es erneut._',
+    errRetryingHint: '_Der Agent wiederholt den Vorgang automatisch, bitte warten ..._',
+    errConfigRequired:
+      'Der aktuelle Konfigurationssatz enthält keine verwendbaren Anmeldedaten. Bitte schließen Sie zunächst die Einrichtung in den API-Einstellungen ab.',
+    startupFailedTitle: 'Open Cowork konnte nicht gestartet werden',
+    startupFailedBody: '{{message}}\n\nWeitere Informationen finden Sie in den Protokollen.',
+    configDefaultSetName: 'Standard',
+    configFallbackSetName: 'Konfiguration {{index}}',
+  },
+  it: {
+    errModelTimeout:
+      "Risposta del modello scaduta: nessuna risposta dal servizio upstream da un po'. Riprova più tardi o controlla il carico attuale del modello/gateway.",
+    errEmptySuccess:
+      'Il modello ha restituito un risultato vuoto pur con esito positivo. Il modello o il gateway attuale potrebbe avere un problema di compatibilità: riprova oppure cambia protocollo e riprova.',
+    errBadRequest:
+      "La richiesta è stata rifiutata dall'upstream (400). La configurazione del modello/protocollo potrebbe essere incompatibile. Controlla il nome del modello, le impostazioni del protocollo e l'endpoint API.\nErrore originale: {{error}}",
+    errAuthFailed:
+      "Autenticazione non riuscita. Controlla se l'API Key è corretta, è scaduta o non ha accesso al modello attuale.\nErrore originale: {{error}}",
+    errRateLimited:
+      "La richiesta è stata sottoposta a limitazione della frequenza (429). Il modello o l'endpoint API attuale ha raggiunto il limite di frequenza delle chiamate. Riprova più tardi.\nErrore originale: {{error}}",
+    errUpstreamError:
+      "Il servizio upstream ha restituito un errore: il servizio del modello potrebbe essere sovraccarico o temporaneamente non disponibile. L'SDK riproverà automaticamente.\nErrore originale: {{error}}",
+    errNetworkInterrupted:
+      'La connessione di rete si è interrotta ({{error}}). Il proxy/gateway potrebbe essere instabile. L’SDK riproverà automaticamente.',
+    errCheckConfigHint: '_Controlla la configurazione e riprova._',
+    errRetryingHint: "_L'agente sta riprovando automaticamente, attendi..._",
+    errConfigRequired:
+      'Il set di configurazione attuale non ha credenziali utilizzabili. Completa prima la configurazione in Impostazioni API.',
+    startupFailedTitle: 'Avvio di Open Cowork non riuscito',
+    startupFailedBody: '{{message}}\n\nControlla i log per maggiori informazioni.',
+    configDefaultSetName: 'Predefinito',
+    configFallbackSetName: 'Configurazione {{index}}',
+  },
+  uk: {
+    errModelTimeout:
+      'Час очікування відповіді моделі вичерпано: вихідний сервіс деякий час не надсилав відповіді. Повторіть спробу пізніше або перевірте поточне навантаження на модель чи шлюз.',
+    errEmptySuccess:
+      'Модель повернула порожній успішний результат. Можливо, поточна модель або шлюз має проблему сумісності — повторіть спробу або змініть протокол і спробуйте знову.',
+    errBadRequest:
+      'Запит відхилено на вихідному сервісі (400). Можливо, конфігурація моделі чи протоколу несумісна. Перевірте назву моделі, налаштування протоколу та точку доступу API.\nПервинна помилка: {{error}}',
+    errAuthFailed:
+      'Помилка автентифікації. Перевірте, чи правильний API Key, чи не сплив його строк дії та чи має він доступ до поточної моделі.\nПервинна помилка: {{error}}',
+    errRateLimited:
+      'Запит обмежено за частотою (429). Поточна модель або точка доступу API досягла ліміту частоти викликів. Повторіть спробу пізніше.\nПервинна помилка: {{error}}',
+    errUpstreamError:
+      'Вихідний сервіс повернув помилку — можливо, сервіс моделі перевантажений або тимчасово недоступний. SDK повторить спробу автоматично.\nПервинна помилка: {{error}}',
+    errNetworkInterrupted:
+      "Мережеве з'єднання було перервано ({{error}}). Можливо, проксі чи шлюз працює нестабільно. SDK повторить спробу автоматично.",
+    errCheckConfigHint: '_Перевірте конфігурацію та повторіть спробу._',
+    errRetryingHint: '_Агент повторює спробу автоматично, зачекайте..._',
+    errConfigRequired:
+      'Поточний набір конфігурації не має придатних облікових даних. Спершу завершіть налаштування в розділі параметрів API.',
+    startupFailedTitle: 'Не вдалося запустити Open Cowork',
+    startupFailedBody: '{{message}}\n\nПерегляньте журнали для отримання додаткової інформації.',
+    configDefaultSetName: 'За замовчуванням',
+    configFallbackSetName: 'Конфігурація {{index}}',
+  },
+  pl: {
+    errModelTimeout:
+      'Przekroczono limit czasu odpowiedzi modelu: usługa nadrzędna od pewnego czasu nie odpowiada. Spróbuj ponownie później lub sprawdź bieżące obciążenie modelu/bramy.',
+    errEmptySuccess:
+      'Model zwrócił pusty wynik mimo powodzenia. Bieżący model lub brama mogą mieć problem ze zgodnością — spróbuj ponownie albo zmień protokół i spróbuj jeszcze raz.',
+    errBadRequest:
+      'Żądanie zostało odrzucone po stronie usługi nadrzędnej (400). Konfiguracja modelu/protokołu może być niezgodna. Sprawdź nazwę modelu, ustawienia protokołu oraz punkt końcowy API.\nBłąd źródłowy: {{error}}',
+    errAuthFailed:
+      'Uwierzytelnianie nie powiodło się. Sprawdź, czy API Key jest poprawny, nie wygasł oraz czy ma dostęp do bieżącego modelu.\nBłąd źródłowy: {{error}}',
+    errRateLimited:
+      'Żądanie zostało ograniczone przez limit szybkości (429). Bieżący model lub punkt końcowy API osiągnął limit liczby wywołań. Spróbuj ponownie później.\nBłąd źródłowy: {{error}}',
+    errUpstreamError:
+      'Usługa nadrzędna zwróciła błąd — usługa modelu może być przeciążona lub chwilowo niedostępna. SDK ponowi próbę automatycznie.\nBłąd źródłowy: {{error}}',
+    errNetworkInterrupted:
+      'Połączenie sieciowe zostało przerwane ({{error}}). Serwer proxy/brama mogą być niestabilne. SDK ponowi próbę automatycznie.',
+    errCheckConfigHint: '_Sprawdź konfigurację i spróbuj ponownie._',
+    errRetryingHint: '_Agent automatycznie ponawia próbę, proszę czekać..._',
+    errConfigRequired:
+      'Bieżący zestaw konfiguracji nie zawiera użytecznych poświadczeń. Najpierw dokończ konfigurację w ustawieniach API.',
+    startupFailedTitle: 'Nie udało się uruchomić Open Cowork',
+    startupFailedBody: '{{message}}\n\nSprawdź dzienniki, aby uzyskać więcej informacji.',
+    configDefaultSetName: 'Domyślny',
+    configFallbackSetName: 'Konfiguracja {{index}}',
+  },
+  sv: {
+    errModelTimeout:
+      'Modellsvaret tog för lång tid: inget svar från uppströmstjänsten på ett tag. Försök igen senare eller kontrollera den aktuella belastningen på modellen/gatewayen.',
+    errEmptySuccess:
+      'Modellen returnerade ett tomt lyckat resultat. Den aktuella modellen eller gatewayen kan ha ett kompatibilitetsproblem – försök igen eller byt protokoll och försök på nytt.',
+    errBadRequest:
+      'Begäran avvisades uppströms (400). Konfigurationen för modellen/protokollet kan vara inkompatibel. Kontrollera modellnamnet, protokollinställningarna och API-slutpunkten.\nUrsprungligt fel: {{error}}',
+    errAuthFailed:
+      'Autentiseringen misslyckades. Kontrollera om din API Key är korrekt, har upphört att gälla eller saknar åtkomst till den aktuella modellen.\nUrsprungligt fel: {{error}}',
+    errRateLimited:
+      'Begäran hastighetsbegränsades (429). Den aktuella modellen eller API-slutpunkten har nått sin gräns för anropsfrekvens. Försök igen senare.\nUrsprungligt fel: {{error}}',
+    errUpstreamError:
+      'Uppströmstjänsten returnerade ett fel – modelltjänsten kan vara överbelastad eller tillfälligt otillgänglig. SDK gör automatiskt ett nytt försök.\nUrsprungligt fel: {{error}}',
+    errNetworkInterrupted:
+      'Nätverksanslutningen avbröts ({{error}}). Proxyn/gatewayen kan vara instabil. SDK gör automatiskt ett nytt försök.',
+    errCheckConfigHint: '_Kontrollera din konfiguration och försök igen._',
+    errRetryingHint: '_Agenten försöker igen automatiskt, vänta..._',
+    errConfigRequired:
+      'Den aktuella konfigurationsuppsättningen saknar användbara autentiseringsuppgifter. Slutför först konfigurationen i API-inställningarna.',
+    startupFailedTitle: 'Open Cowork kunde inte starta',
+    startupFailedBody: '{{message}}\n\nKontrollera loggarna för mer information.',
+    configDefaultSetName: 'Standard',
+    configFallbackSetName: 'Konfiguration {{index}}',
+  },
+  no: {
+    errModelTimeout:
+      'Tidsavbrudd for modellsvaret: ingen respons fra den underliggende tjenesten på en stund. Prøv igjen senere, eller sjekk gjeldende belastning på modellen/gatewayen.',
+    errEmptySuccess:
+      'Modellen returnerte et tomt, vellykket resultat. Gjeldende modell eller gateway kan ha et kompatibilitetsproblem – prøv igjen, eller bytt protokoll og prøv på nytt.',
+    errBadRequest:
+      'Forespørselen ble avvist av den underliggende tjenesten (400). Modell-/protokollkonfigurasjonen kan være inkompatibel. Sjekk modellnavnet, protokollinnstillingene og API-endepunktet.\nOpprinnelig feil: {{error}}',
+    errAuthFailed:
+      'Autentiseringen mislyktes. Sjekk om API Key er riktig, har utløpt, eller mangler tilgang til gjeldende modell.\nOpprinnelig feil: {{error}}',
+    errRateLimited:
+      'Forespørselen ble begrenset på grunn av for høy frekvens (429). Gjeldende modell eller API-endepunkt har nådd grensen for antall kall. Prøv igjen senere.\nOpprinnelig feil: {{error}}',
+    errUpstreamError:
+      'Den underliggende tjenesten returnerte en feil – modelltjenesten kan være overbelastet eller midlertidig utilgjengelig. SDK prøver automatisk på nytt.\nOpprinnelig feil: {{error}}',
+    errNetworkInterrupted:
+      'Nettverkstilkoblingen ble avbrutt ({{error}}). Proxyen/gatewayen kan være ustabil. SDK prøver automatisk på nytt.',
+    errCheckConfigHint: '_Sjekk konfigurasjonen og prøv igjen._',
+    errRetryingHint: '_Agenten prøver automatisk på nytt, vent litt …_',
+    errConfigRequired:
+      'Gjeldende konfigurasjonssett har ingen brukbare legitimasjoner. Fullfør oppsettet i API-innstillinger først.',
+    startupFailedTitle: 'Open Cowork kunne ikke starte',
+    startupFailedBody: '{{message}}\n\nSjekk loggene for mer informasjon.',
+    configDefaultSetName: 'Standard',
+    configFallbackSetName: 'Konfigurasjon {{index}}',
+  },
+  nl: {
+    errModelTimeout:
+      'Time-out bij modelantwoord: het upstream-service reageerde een tijd lang niet. Probeer het later opnieuw of controleer de huidige belasting van het model/de gateway.',
+    errEmptySuccess:
+      'Het model gaf een leeg succesvol resultaat terug. Mogelijk is er een compatibiliteitsprobleem met het huidige model of de gateway — probeer het opnieuw of schakel over op een ander protocol en probeer het nogmaals.',
+    errBadRequest:
+      'De aanvraag werd upstream geweigerd (400). Mogelijk is de model-/protocolconfiguratie niet compatibel. Controleer de modelnaam, de protocolinstellingen en het API-eindpunt.\nOorspronkelijke fout: {{error}}',
+    errAuthFailed:
+      'Verificatie mislukt. Controleer of de API Key juist is, niet verlopen is en toegang heeft tot het huidige model.\nOorspronkelijke fout: {{error}}',
+    errRateLimited:
+      'De aanvraag is gelimiteerd (429). Het huidige model of API-eindpunt heeft zijn limiet voor het aantal aanroepen bereikt. Probeer het later opnieuw.\nOorspronkelijke fout: {{error}}',
+    errUpstreamError:
+      'Het upstream-service gaf een fout terug — de modelservice is mogelijk overbelast of tijdelijk niet beschikbaar. De SDK probeert het automatisch opnieuw.\nOorspronkelijke fout: {{error}}',
+    errNetworkInterrupted:
+      'De netwerkverbinding werd onderbroken ({{error}}). De proxy/gateway is mogelijk instabiel. De SDK probeert het automatisch opnieuw.',
+    errCheckConfigHint: '_Controleer je configuratie en probeer het opnieuw._',
+    errRetryingHint: '_De agent probeert het automatisch opnieuw, even geduld..._',
+    errConfigRequired:
+      'De huidige configuratieset bevat geen bruikbare inloggegevens. Voltooi eerst de installatie in de API-instellingen.',
+    startupFailedTitle: 'Open Cowork kon niet worden gestart',
+    startupFailedBody: '{{message}}\n\nRaadpleeg de logbestanden voor meer informatie.',
+    configDefaultSetName: 'Standaard',
+    configFallbackSetName: 'Configuratie {{index}}',
+  },
+  ro: {
+    errModelTimeout:
+      'Răspunsul modelului a expirat: niciun răspuns de la serviciul din amonte pentru o vreme. Reîncearcă mai târziu sau verifică gradul de încărcare al modelului/gateway-ului.',
+    errEmptySuccess:
+      'Modelul a returnat un rezultat reușit, dar gol. Este posibil ca modelul sau gateway-ul curent să aibă o problemă de compatibilitate — reîncearcă sau schimbă protocolul și încearcă din nou.',
+    errBadRequest:
+      'Cererea a fost respinsă în amonte (400). Configurația modelului/protocolului poate fi incompatibilă. Verifică numele modelului, setările de protocol și punctul de acces API.\nEroare originală: {{error}}',
+    errAuthFailed:
+      'Autentificarea a eșuat. Verifică dacă API Key este corectă, a expirat sau nu are acces la modelul curent.\nEroare originală: {{error}}',
+    errRateLimited:
+      'Cererea a fost limitată ca rată (429). Modelul sau punctul de acces API curent a atins limita de rată a apelurilor. Reîncearcă mai târziu.\nEroare originală: {{error}}',
+    errUpstreamError:
+      'Serviciul din amonte a returnat o eroare — este posibil ca serviciul modelului să fie suprasolicitat sau temporar indisponibil. SDK va reîncerca automat.\nEroare originală: {{error}}',
+    errNetworkInterrupted:
+      'Conexiunea la rețea a fost întreruptă ({{error}}). Este posibil ca proxy-ul/gateway-ul să fie instabil. SDK va reîncerca automat.',
+    errCheckConfigHint: '_Verifică configurația și reîncearcă._',
+    errRetryingHint: '_Agentul reîncearcă automat, te rugăm să aștepți..._',
+    errConfigRequired:
+      'Setul de configurație curent nu are credențiale utilizabile. Finalizează mai întâi configurarea în Setări API.',
+    startupFailedTitle: 'Open Cowork nu a putut porni',
+    startupFailedBody: '{{message}}\n\nVerifică jurnalele pentru mai multe informații.',
+    configDefaultSetName: 'Implicit',
+    configFallbackSetName: 'Configurația {{index}}',
+  },
+};
diff --git a/src/main/i18n/index.ts b/src/main/i18n/index.ts
new file mode 100644
index 00000000..e423de01
--- /dev/null
+++ b/src/main/i18n/index.ts
@@ -0,0 +1,52 @@
+// Main-process (backend) translator.
+//
+// The renderer owns the active UI language (react-i18next) and mirrors it into
+// the config store as `uiLanguage`; the main process reads that value and calls
+// `setBackendLanguage()` so backend-produced strings (errors, dialogs, the
+// default config-set name) match what the user picked. Chinese is the default
+// and the ultimate fallback — the product is Chinese-first.
+
+import { backendCatalog, DEFAULT_BACKEND_LANGUAGE, type BackendMessageKey } from './catalog';
+
+export { DEFAULT_BACKEND_LANGUAGE } from './catalog';
+export type { BackendMessageKey } from './catalog';
+
+let currentLanguage: string = DEFAULT_BACKEND_LANGUAGE;
+
+/**
+ * Normalize an arbitrary locale tag to one of our catalog keys:
+ * strip the region (`es-ES` -> `es`, `zh-CN` -> `zh`) and fold the Norwegian
+ * variants (`nb`/`nn`) onto the shipped `no` table. Returns the default
+ * language when the result isn't in the catalog.
+ */
+function normalizeBackendLanguage(lang: string | undefined | null): string {
+  if (!lang) return DEFAULT_BACKEND_LANGUAGE;
+  const base = lang.toLowerCase().split(/[-_]/)[0];
+  const folded = base === 'nb' || base === 'nn' ? 'no' : base;
+  return backendCatalog[folded] ? folded : DEFAULT_BACKEND_LANGUAGE;
+}
+
+/** Update the language used by mt(). Safe to call repeatedly. */
+export function setBackendLanguage(lang: string | undefined | null): void {
+  currentLanguage = normalizeBackendLanguage(lang);
+}
+
+/** The language mt() is currently translating into. */
+export function getBackendLanguage(): string {
+  return currentLanguage;
+}
+
+/**
+ * Translate a backend message key into the active language, falling back to the
+ * default (Chinese) table and finally to the key itself. `params` are
+ * interpolated into `{{name}}` placeholders.
+ */
+export function mt(key: BackendMessageKey, params?: Record<string, string | number>): string {
+  const table = backendCatalog[currentLanguage] ?? backendCatalog[DEFAULT_BACKEND_LANGUAGE];
+  const template =
+    table?.[key] ?? backendCatalog[DEFAULT_BACKEND_LANGUAGE]?.[key] ?? (key as string);
+  if (!params) return template;
+  return template.replace(/\{\{(\w+)\}\}/g, (match, name: string) =>
+    name in params ? String(params[name]) : match
+  );
+}
diff --git a/src/main/index.ts b/src/main/index.ts
index 3567ad44..2a5c513d 100644
--- a/src/main/index.ts
+++ b/src/main/index.ts
@@ -32,6 +32,7 @@ import {
   type AppTheme,
   type CreateConfigSetPayload,
 } from './config/config-store';
+import { mt } from './i18n';
 import { runConfigApiTest } from './config/config-test-routing';
 import { listOllamaModels } from './config/ollama-api';
 import { setPermissionRules } from './config/permission-rules-store';
@@ -1007,7 +1008,7 @@ app
   .catch((error) => {
     logError('[App] Startup failed:', error);
     const message = error instanceof Error ? error.message : 'Unknown startup error';
-    dialog.showErrorBox('Open Cowork 启动失败', `${message}\n\n请查看日志获取更多信息。`);
+    dialog.showErrorBox(mt('startupFailedTitle'), mt('startupFailedBody', { message }));
     app.quit();
   });
 
@@ -2665,7 +2666,7 @@ async function handleClientEvent(event: ClientEvent): Promise<unknown> {
     sendToRenderer({
       type: 'error',
       payload: {
-        message: '当前方案未配置可用凭证，请先在 API 设置中完成配置',
+        message: mt('errConfigRequired'),
         code: 'CONFIG_REQUIRED_ACTIVE_SET',
         action: 'open_api_settings',
       },
diff --git a/src/renderer/i18n/config.ts b/src/renderer/i18n/config.ts
index 6855c8e5..f6dd504c 100644
--- a/src/renderer/i18n/config.ts
+++ b/src/renderer/i18n/config.ts
@@ -5,7 +5,7 @@ import LanguageDetector from 'i18next-browser-languagedetector';
 import enTranslations from './locales/en.json';
 import zhTranslations from './locales/zh.json';
 
-i18n
+const initPromise = i18n
   .use(LanguageDetector) // 自动检测浏览器语言
   .use(initReactI18next) // 初始化 react-i18next
   .init({
@@ -31,4 +31,22 @@ i18n
     },
   });
 
+// 将渲染进程的语言镜像到主进程配置，使后端字符串（错误、对话框）使用同一语言。
+// Mirror the renderer language into the main-process config so backend-produced
+// strings (errors, dialogs, the default config-set name) match the UI. Fires for
+// the initially detected language and whenever the user switches.
+let lastSyncedLanguage: string | undefined;
+function syncBackendLanguage(lng?: string): void {
+  if (!lng || lng === lastSyncedLanguage) return;
+  lastSyncedLanguage = lng;
+  try {
+    void window.electronAPI?.config?.save?.({ uiLanguage: lng });
+  } catch {
+    /* ignore: browser/dev mode without electronAPI */
+  }
+}
+
+i18n.on('languageChanged', syncBackendLanguage);
+void initPromise.then(() => syncBackendLanguage(i18n.language));
+
 export default i18n;
diff --git a/src/renderer/types/index.ts b/src/renderer/types/index.ts
index 7b3c03b4..c4534fff 100644
--- a/src/renderer/types/index.ts
+++ b/src/renderer/types/index.ts
@@ -83,8 +83,8 @@ export interface ToolResultContent {
   content: string;
   isError?: boolean;
   images?: Array<{
-    data: string;          // base64 encoded image data
-    mimeType: string;      // e.g., 'image/png'
+    data: string; // base64 encoded image data
+    mimeType: string; // e.g., 'image/png'
   }>;
 }
 
@@ -479,17 +479,17 @@ export type ClientEvent =
   | { type: 'workdir.select'; payload: { sessionId?: string; currentPath?: string } };
 
 // Sandbox setup types (app startup)
-export type SandboxSetupPhase = 
-  | 'checking'      // Checking WSL/Lima availability
-  | 'creating'      // Creating Lima instance (macOS only)
-  | 'starting'      // Starting Lima instance (macOS only)  
+export type SandboxSetupPhase =
+  | 'checking' // Checking WSL/Lima availability
+  | 'creating' // Creating Lima instance (macOS only)
+  | 'starting' // Starting Lima instance (macOS only)
   | 'installing_node' // Installing Node.js
   | 'installing_python' // Installing Python
-  | 'installing_pip'    // Installing pip
-  | 'installing_deps'   // Installing skill dependencies (markitdown, pypdf, etc.)
-  | 'ready'         // Ready to use
-  | 'skipped'       // No sandbox needed (native mode)
-  | 'error';        // Setup failed
+  | 'installing_pip' // Installing pip
+  | 'installing_deps' // Installing skill dependencies (markitdown, pypdf, etc.)
+  | 'ready' // Ready to use
+  | 'skipped' // No sandbox needed (native mode)
+  | 'error'; // Setup failed
 
 export interface SandboxSetupProgress {
   phase: SandboxSetupPhase;
@@ -501,11 +501,11 @@ export interface SandboxSetupProgress {
 
 // Sandbox sync types (per-session file sync)
 export type SandboxSyncPhase =
-  | 'starting_agent'  // Starting WSL/Lima agent
-  | 'syncing_files'   // Syncing files to sandbox
+  | 'starting_agent' // Starting WSL/Lima agent
+  | 'syncing_files' // Syncing files to sandbox
   | 'syncing_skills' // Copying skills
-  | 'ready'           // Sync complete
-  | 'error';          // Sync failed
+  | 'ready' // Sync complete
+  | 'error'; // Sync failed
 
 export interface SandboxSyncStatus {
   sessionId: string;
@@ -520,8 +520,14 @@ export type ServerEvent =
   | { type: 'stream.message'; payload: { sessionId: string; message: Message } }
   | { type: 'stream.partial'; payload: { sessionId: string; delta: string } }
   | { type: 'stream.thinking'; payload: { sessionId: string; delta: string } }
-  | { type: 'stream.executionTime'; payload: { sessionId: string; messageId: string; executionTimeMs: number } }
-  | { type: 'session.status'; payload: { sessionId: string; status: SessionStatus; error?: string } }
+  | {
+      type: 'stream.executionTime';
+      payload: { sessionId: string; messageId: string; executionTimeMs: number };
+    }
+  | {
+      type: 'session.status';
+      payload: { sessionId: string; status: SessionStatus; error?: string };
+    }
   | { type: 'session.update'; payload: { sessionId: string; updates: Partial<Session> } }
   | { type: 'session.list'; payload: { sessions: Session[] } }
   | { type: 'permission.request'; payload: PermissionRequest }
@@ -529,21 +535,37 @@ export type ServerEvent =
   | { type: 'sudo.password.request'; payload: SudoPasswordRequest }
   | { type: 'sudo.password.dismiss'; payload: { toolUseId: string } }
   | { type: 'trace.step'; payload: { sessionId: string; step: TraceStep } }
-  | { type: 'trace.update'; payload: { sessionId: string; stepId: string; updates: Partial<TraceStep> } }
+  | {
+      type: 'trace.update';
+      payload: { sessionId: string; stepId: string; updates: Partial<TraceStep> };
+    }
   | { type: 'folder.selected'; payload: { path: string } }
   | { type: 'config.status'; payload: { isConfigured: boolean; config: AppConfig } }
   | { type: 'sandbox.progress'; payload: SandboxSetupProgress }
   | { type: 'sandbox.sync'; payload: SandboxSyncStatus }
   | { type: 'skills.storageChanged'; payload: SkillsStorageChangeEvent }
-  | { type: 'plugins.runtimeApplied'; payload: { sessionId: string; plugins: Array<{ name: string; path: string }> } }
+  | {
+      type: 'plugins.runtimeApplied';
+      payload: { sessionId: string; plugins: Array<{ name: string; path: string }> };
+    }
   | { type: 'workdir.changed'; payload: { path: string } }
   | { type: 'session.contextInfo'; payload: { sessionId: string; contextWindow: number } }
-  | { type: 'navigate.to'; payload: { page: 'welcome' | 'settings' | 'session'; tab?: string; sessionId?: string } }
+  | {
+      type: 'navigate.to';
+      payload: { page: 'welcome' | 'settings' | 'session'; tab?: string; sessionId?: string };
+    }
   | { type: 'native-theme.changed'; payload: { shouldUseDarkColors: boolean } }
   | { type: 'new-session' }
   | { type: 'navigate'; payload: string }
   | { type: 'scheduled-task.error'; payload: { taskId: string; error: string } }
-  | { type: 'error'; payload: { message: string; code?: 'CONFIG_REQUIRED_ACTIVE_SET'; action?: 'open_api_settings' } };
+  | {
+      type: 'error';
+      payload: {
+        message: string;
+        code?: 'CONFIG_REQUIRED_ACTIVE_SET';
+        action?: 'open_api_settings';
+      };
+    };
 
 // Settings types
 export interface Settings {
@@ -557,7 +579,15 @@ export interface Settings {
 }
 
 // Tool types
-export type ToolName = 'read' | 'write' | 'edit' | 'glob' | 'grep' | 'bash' | 'webFetch' | 'webSearch';
+export type ToolName =
+  | 'read'
+  | 'write'
+  | 'edit'
+  | 'glob'
+  | 'grep'
+  | 'bash'
+  | 'webFetch'
+  | 'webSearch';
 
 export interface ToolResult {
   success: boolean;
@@ -654,6 +684,7 @@ export interface AppConfig {
   defaultWorkdir?: string;
   globalSkillsPath?: string;
   theme?: AppTheme;
+  uiLanguage?: string;
   sandboxEnabled?: boolean;
   memoryEnabled?: boolean;
   memoryRuntime?: MemoryRuntimeConfig;
@@ -753,10 +784,7 @@ export interface LocalServiceInfo {
   models?: string[];
 }
 
-export type LocalOllamaDiscoveryStatus =
-  | 'unavailable'
-  | 'service_available'
-  | 'models_available';
+export type LocalOllamaDiscoveryStatus = 'unavailable' | 'service_available' | 'models_available';
 
 export interface LocalOllamaDiscoveryResult {
   available: boolean;
diff --git a/src/tests/i18n/backend-i18n.test.ts b/src/tests/i18n/backend-i18n.test.ts
new file mode 100644
index 00000000..4f0c5e44
--- /dev/null
+++ b/src/tests/i18n/backend-i18n.test.ts
@@ -0,0 +1,77 @@
+import { afterEach, describe, expect, it } from 'vitest';
+
+import {
+  backendCatalog,
+  DEFAULT_BACKEND_LANGUAGE,
+  SUPPORTED_BACKEND_LANGUAGES,
+  type BackendMessageKey,
+} from '../../main/i18n/catalog';
+import { getBackendLanguage, mt, setBackendLanguage } from '../../main/i18n';
+
+// setBackendLanguage mutates module state; restore the default after each test.
+afterEach(() => setBackendLanguage(DEFAULT_BACKEND_LANGUAGE));
+
+describe('backend i18n catalog', () => {
+  const zhKeys = Object.keys(backendCatalog.zh).sort();
+
+  it('defaults to Chinese (the project is Chinese-first)', () => {
+    expect(DEFAULT_BACKEND_LANGUAGE).toBe('zh');
+  });
+
+  it('every supported language has exactly the same keys as the zh source', () => {
+    for (const lang of SUPPORTED_BACKEND_LANGUAGES) {
+      expect(backendCatalog[lang], `missing table for ${lang}`).toBeDefined();
+      expect(Object.keys(backendCatalog[lang]).sort(), `key drift in ${lang}`).toEqual(zhKeys);
+    }
+  });
+
+  it('preserves placeholders and underscores in every translation', () => {
+    for (const lang of SUPPORTED_BACKEND_LANGUAGES) {
+      const table = backendCatalog[lang];
+      expect(table.errBadRequest, `${lang} errBadRequest`).toContain('{{error}}');
+      expect(table.startupFailedBody, `${lang} startupFailedBody`).toContain('{{message}}');
+      expect(table.configFallbackSetName, `${lang} configFallbackSetName`).toContain('{{index}}');
+      expect(table.errCheckConfigHint.startsWith('_'), `${lang} hint italics`).toBe(true);
+      expect(table.errCheckConfigHint.endsWith('_'), `${lang} hint italics`).toBe(true);
+    }
+  });
+});
+
+describe('mt()', () => {
+  it('translates into the active language', () => {
+    setBackendLanguage('en');
+    expect(getBackendLanguage()).toBe('en');
+    expect(mt('startupFailedTitle')).toBe('Open Cowork failed to start');
+    setBackendLanguage('zh');
+    expect(mt('startupFailedTitle')).toBe('Open Cowork 启动失败');
+  });
+
+  it('interpolates {{params}} and leaves none behind', () => {
+    setBackendLanguage('en');
+    const out = mt('errBadRequest', { error: 'boom-detail' });
+    expect(out).toContain('boom-detail');
+    expect(out).not.toContain('{{error}}');
+  });
+
+  it('normalizes region variants and Norwegian forms to a catalog table', () => {
+    setBackendLanguage('es-ES');
+    expect(getBackendLanguage()).toBe('es');
+    setBackendLanguage('zh-CN');
+    expect(getBackendLanguage()).toBe('zh');
+    setBackendLanguage('nb-NO');
+    expect(getBackendLanguage()).toBe('no');
+    setBackendLanguage('nn');
+    expect(getBackendLanguage()).toBe('no');
+  });
+
+  it('falls back to the default language for unknown locales', () => {
+    setBackendLanguage('xx-YY');
+    expect(getBackendLanguage()).toBe(DEFAULT_BACKEND_LANGUAGE);
+    expect(mt('configDefaultSetName')).toBe(backendCatalog.zh.configDefaultSetName);
+  });
+
+  it('falls back to the key name for an unknown key', () => {
+    setBackendLanguage('en');
+    expect(mt('not_a_real_key' as BackendMessageKey)).toBe('not_a_real_key');
+  });
+});