The French texts seem to work fine here on Meta, so I don’t think it’s a problem with the French translation in general.
Based on the errors in the browser console when looking at your forum, it seems loading the additional locale files fails.
Access to script at ‘https://forumfrancaisalondres.b-cdn.net//extra-locales/6a7897ea231de2274b38eb816410d3f1ac2fe4e4/fr/main.js’ from origin ‘https://forum.francaisalondres.com’ has been blocked by CORS policy: No ‘Access-Control-Allow-Origin’ header is present on the requested resource.