Sorry for the delay. I just:
* Went to your homepage
* Used the switcher in the bottom right to set my language to French
* Registered using the Facebook button on the left hand side of the main home page area
* After doing that, I was brought to:
/fr/hom.html
So, in that quick test, my language was kept at French. Can you tell me how you're testing?
Also, let me know what your New User and Returning User redirection settings are. If New User Registration is "Same Page" and "Returning User" is set to a specific menu item, that would likely explain the problem.
Let me know any more information you may have, and we'll gladly help however we can.
Thanks,
Alex