- Create i18n utility module (app/utils/i18n.py) with translation loading,
browser language detection, AI fallback, and l10n helpers
- Add JSON translation files for EN, DE, FR, ES, IT, PT, NL, PL, ZH, RU
- Add preferred_language column to UserProfile model with migration
- Register _() translation function as Jinja2 global
- Update base.html with translated navigation, footer, cookie notice
- Add language selector dropdown in nav bar (desktop + mobile)
- Create API endpoints for language preference (POST/GET /api/i18n/)
- Support language detection: user profile > cookie > Accept-Language > default
Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>