[FEATURE] Add French (fr) as a UI language #405

Is your feature request related to a problem? Please describe

CARE's interface is available in English and German. French-speaking users have no option in their language, so they fall back to English.

Describe the solution you'd like

Add French as a third UI language, following section 5 ("Adding a new UI language") of docs/source/for_developers/i18n.rst:

Describe alternatives you've considered

Additional context

• Translations are drafted with LLM assistance and reviewed entry by entry by a human before merging.
• Out of scope, per the i18n guidelines: table content, server logs, developer console output and template body content stay untranslated.