Auto-detect LLM output language from system locale on first startup

packages/cli/src/core/initializer.ts

@@ -15,6 +15,7 @@ import { type LoadedSettings, SettingScope } from '../config/settings.js';
 import { performInitialAuth } from './auth.js';
 import { validateTheme } from './theme.js';
 import { initializeI18n } from '../i18n/index.js';
+import { initializeLlmOutputLanguage } from '../ui/commands/languageCommand.js';
 
 export interface InitializationResult {
   authError: string | null;
@@ -41,6 +42,9 @@ export async function initializeApp(
     'auto';
   await initializeI18n(languageSetting);
 
+  // Auto-detect and set LLM output language on first use
+  initializeLlmOutputLanguage();
+
   const authType = settings.merged.security?.auth?.selectedType;
   const authError = await performInitialAuth(config, authType);
 

packages/cli/src/i18n/index.ts

@@ -46,17 +46,27 @@ const getLocalePath = (
   return path.join(baseDir, `${lang}.js`);
 };
 
+// Supported locale codes mapped to English language names
+const LOCALE_TO_LANGUAGE_NAME: Record<string, string> = {
+  zh: 'Chinese',
+  en: 'English',
+  ru: 'Russian',
+  de: 'German',
+};
+
 // Language detection
 export function detectSystemLanguage(): SupportedLanguage {
   const envLang = process.env['QWEN_CODE_LANG'] || process.env['LANG'];
   if (envLang?.startsWith('zh')) return 'zh';
   if (envLang?.startsWith('en')) return 'en';
   if (envLang?.startsWith('ru')) return 'ru';
+  if (envLang?.startsWith('de')) return 'de';
 
   try {
     const locale = Intl.DateTimeFormat().resolvedOptions().locale;
     if (locale.startsWith('zh')) return 'zh';
     if (locale.startsWith('ru')) return 'ru';
+    if (locale.startsWith('de')) return 'de';
   } catch {
     // Fallback to default
   }
@@ -64,6 +74,14 @@ export function detectSystemLanguage(): SupportedLanguage {
   return 'en';
 }
 
+/**
+ * Maps a locale code to its English language name.
+ * Used for LLM output language instructions.
+ */
+export function getLanguageNameFromLocale(locale: SupportedLanguage): string {
+  return LOCALE_TO_LANGUAGE_NAME[locale] || 'English';
+}
+
 // Translation loading
 async function loadTranslationsAsync(
   lang: SupportedLanguage,

packages/cli/src/ui/commands/languageCommand.test.ts

@@ -13,6 +13,16 @@ import { createMockCommandContext } from '../../test-utils/mockCommandContext.js
 vi.mock('../../i18n/index.js', () => ({
   setLanguageAsync: vi.fn().mockResolvedValue(undefined),
   getCurrentLanguage: vi.fn().mockReturnValue('en'),
+  detectSystemLanguage: vi.fn().mockReturnValue('en'),
+  getLanguageNameFromLocale: vi.fn((locale: string) => {
+    const map: Record<string, string> = {
+      zh: 'Chinese',
+      en: 'English',
+      ru: 'Russian',
+      de: 'German',
+    };
+    return map[locale] || 'English';
+  }),
   t: vi.fn((key: string) => key),
 }));
 
@@ -61,7 +71,10 @@ vi.mock('@qwen-code/qwen-code-core', async (importOriginal) => {
 
 // Import modules after mocking
 import * as i18n from '../../i18n/index.js';
-import { languageCommand } from './languageCommand.js';
+import {
+  languageCommand,
+  initializeLlmOutputLanguage,
+} from './languageCommand.js';
 
 describe('languageCommand', () => {
   let mockContext: CommandContext;
@@ -597,4 +610,74 @@ describe('languageCommand', () => {
       });
     });
   });
+
+  describe('initializeLlmOutputLanguage', () => {
+    beforeEach(() => {
+      vi.clearAllMocks();
+      vi.mocked(fs.existsSync).mockReturnValue(false);
+      vi.mocked(fs.mkdirSync).mockImplementation(() => undefined);
+      vi.mocked(fs.writeFileSync).mockImplementation(() => undefined);
+    });
+
+    it('should create file when it does not exist', () => {
+      vi.mocked(fs.existsSync).mockReturnValue(false);
+      vi.mocked(i18n.detectSystemLanguage).mockReturnValue('en');
+
+      initializeLlmOutputLanguage();
+
+      expect(fs.mkdirSync).toHaveBeenCalled();
+      expect(fs.writeFileSync).toHaveBeenCalledWith(
+        expect.stringContaining('output-language.md'),
+        expect.stringContaining('English'),
+        'utf-8',
+      );
+    });
+
+    it('should NOT overwrite existing file', () => {
+      vi.mocked(fs.existsSync).mockReturnValue(true);
+
+      initializeLlmOutputLanguage();
+
+      expect(fs.writeFileSync).not.toHaveBeenCalled();
+    });
+
+    it('should detect Chinese locale and create Chinese rule file', () => {
+      vi.mocked(fs.existsSync).mockReturnValue(false);
+      vi.mocked(i18n.detectSystemLanguage).mockReturnValue('zh');
+
+      initializeLlmOutputLanguage();
+
+      expect(fs.writeFileSync).toHaveBeenCalledWith(
+        expect.stringContaining('output-language.md'),
+        expect.stringContaining('Chinese'),
+        'utf-8',
+      );
+    });
+
+    it('should detect Russian locale and create Russian rule file', () => {
+      vi.mocked(fs.existsSync).mockReturnValue(false);
+      vi.mocked(i18n.detectSystemLanguage).mockReturnValue('ru');
+
+      initializeLlmOutputLanguage();
+
+      expect(fs.writeFileSync).toHaveBeenCalledWith(
+        expect.stringContaining('output-language.md'),
+        expect.stringContaining('Russian'),
+        'utf-8',
+      );
+    });
+
+    it('should detect German locale and create German rule file', () => {
+      vi.mocked(fs.existsSync).mockReturnValue(false);
+      vi.mocked(i18n.detectSystemLanguage).mockReturnValue('de');
+
+      initializeLlmOutputLanguage();
+
+      expect(fs.writeFileSync).toHaveBeenCalledWith(
+        expect.stringContaining('output-language.md'),
+        expect.stringContaining('German'),
+        'utf-8',
+      );
+    });
+  });
 });

packages/cli/src/ui/commands/languageCommand.ts

@@ -15,6 +15,8 @@ import { SettingScope } from '../../config/settings.js';
 import {
   setLanguageAsync,
   getCurrentLanguage,
+  detectSystemLanguage,
+  getLanguageNameFromLocale,
   type SupportedLanguage,
   t,
 } from '../../i18n/index.js';
@@ -73,6 +75,33 @@ function getLlmOutputLanguageRulePath(): string {
   );
 }
 
+/**
+ * Initializes the LLM output language rule file on first startup.
+ * If the file already exists, it is not overwritten (respects user preference).
+ */
+export function initializeLlmOutputLanguage(): void {
+  const filePath = getLlmOutputLanguageRulePath();
+
+  // Skip if file already exists (user preference)
+  if (fs.existsSync(filePath)) {
+    return;
+  }
+
+  // Detect system language and map to language name
+  const detectedLocale = detectSystemLanguage();
+  const languageName = getLanguageNameFromLocale(detectedLocale);
+
+  // Generate the rule file
+  const content = generateLlmOutputLanguageRule(languageName);
+
+  // Ensure directory exists
+  const dir = path.dirname(filePath);
+  fs.mkdirSync(dir, { recursive: true });
+
+  // Write file
+  fs.writeFileSync(filePath, content, 'utf-8');
+}
+
 /**
  * Gets the current LLM output language from the rule file if it exists.
  */