Translation and CSS

Jérémy,
Unfortunately, there are big limitations as to how you can style the widgets from Facebook. They don't allow user's to include their own CSS or modify the overall layout. That's to prevent abuse in ways to completely remove identifications of Facebook, which has happened in the past when they had looser restrictions. Most modules do have a "Light" or "Dark" colorscheme though, which should give some options as far as how the content is displayed.

As for the French translations, unfortunately, no one has contributed the back-end strings yet. We have an area on Transifex where you're welcome to contribute, or if you would rather post any translations here, we can get them updated for the next release:
www.transifex.com/projects/p/jfbconnect/language/fr_FR/

Hope that helps,
Alex