Propagate changes from main into development

[github-actions]

Automated PR to propagate changes from main into development.

Triggered by push to main.

[Copilot]

Pull request overview

This PR propagates i18n (internationalization) implementation changes from main into development, adding comprehensive multi-language support to the Charon application with 5 languages (English, Spanish, French, German, and Chinese).

Key Changes:

• Complete i18n infrastructure using i18next with automatic language detection and localStorage persistence
• 5 translation files with 130+ keys each covering UI elements, navigation, errors, and notifications
• New LanguageSelector component integrated into System Settings for instant language switching

Reviewed changes

Copilot reviewed 21 out of 22 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
frontend/src/i18n.ts	Core i18n configuration with language detection and fallback settings
frontend/src/context/LanguageContext.tsx	React context provider for language state management
frontend/src/context/LanguageContextValue.ts	TypeScript types for language context
frontend/src/hooks/useLanguage.ts	Custom hook for accessing language context
frontend/src/components/LanguageSelector.tsx	UI component for language selection dropdown
frontend/src/main.tsx	Integration of LanguageProvider into app root
frontend/src/pages/SystemSettings.tsx	Addition of language selector to settings page
frontend/src/locales/*/translation.json	Translation files for 5 languages (en, es, fr, de, zh)
frontend/src/__tests__/i18n.test.ts	Unit tests for i18n configuration
frontend/src/hooks/__tests__/useLanguage.test.tsx	Unit tests for language hook
frontend/src/components/__tests__/LanguageSelector.test.tsx	Unit tests for language selector component
frontend/src/pages/__tests__/SystemSettings.test.tsx	Updated tests with LanguageProvider and i18next mock
frontend/package.json	Added i18next dependencies (i18next, react-i18next, i18next-browser-languagedetector)
frontend/package-lock.json	Dependency lock file updates for new packages
CONTRIBUTING_TRANSLATIONS.md	Comprehensive guide for contributing translations
docs/i18n-examples.md	Developer guide with implementation examples
docs/features.md	Updated with multi-language feature documentation
I18N_IMPLEMENTATION_SUMMARY.md	Complete implementation summary and metrics

Files not reviewed (1)

• frontend/package-lock.json: Language not supported

[Copilot]

The setLanguage function calls i18n.changeLanguage twice unnecessarily. When setLanguage is called, it invokes i18n.changeLanguage on line 19. Then, the useEffect hook on lines 12-14 detects the state change and calls i18n.changeLanguage again. This creates redundant language change operations. Remove the i18n.changeLanguage call from the setLanguage function since the useEffect will handle the language change whenever the state updates.

Suggested change

i18n.changeLanguage(lang)

[Copilot]

The saved language from localStorage is not validated before use. If a user manually modifies localStorage to contain an invalid language code, it will be cast to the Language type without validation, potentially causing runtime errors. Add validation to ensure the saved value is one of the supported languages before using it.

Suggested change

	export function LanguageProvider({ children }: { children: ReactNode }) {
	const { i18n } = useTranslation()
	const [language, setLanguageState] = useState<Language>(() => {
	const saved = localStorage.getItem('charon-language')
	return (saved as Language) || 'en'
	const SUPPORTED_LANGUAGES: Language[] = ['en', 'es', 'fr', 'de', 'zh']
	export function LanguageProvider({ children }: { children: ReactNode }) {
	const { i18n } = useTranslation()
	const [language, setLanguageState] = useState<Language>(() => {
	const saved = localStorage.getItem('charon-language')
	if (saved && (SUPPORTED_LANGUAGES as readonly string[]).includes(saved)) {
	return saved as Language
	}
	return 'en'