For information, the extension now uses new locale registration APIs (implemented here in Flarum's core).
As those locale registration APIs doesn't exist in the released Flarum beta (and development releases older than this change), the extension may not works with you current installation. So, if you're in this situation, you have to manually reverse this change in the extensions/french/src/Extension.php file located on your server.
Another information, according to Toby on Gitter, the whole structure for localizations will be getting a revamp soon, so be prepared. I'll try to be as quick as possible to update the extension when this revamp will be done.