Added French Language Support

French is now a supported language in Basker. Support covers CMS labels, content locale handling, preview state, and runtime language switching. The language picker and middleware keep locale state current as editors move between previews and languages, so switching from English to French mid-session does not leave stale interface strings or a broken preview context.

Adding more languages

The same locale handling applies to any language added later. A new language needs labels, route validation, preview state, date formatting, and theme data to agree before it works in production, and those pieces are now shared rather than built per language.