Problem: language preference stored only in localStorage, lost on 'clear site data'. French users with browser lang=fr always got French UI even when profile was English. Solution: - Backend: add 'language' field to user profile (default 'fr') - Backend: PATCH /api/auth/me to update language preference - Frontend: initI18n() priority: server > localStorage > navigator > 'fr' - Frontend: initAuth() runs before initI18n() so server check works - Frontend: profile Save persists language to server via PATCH Verified: tabs show English after server profile set to 'en'