The french version of the game shouldn't require a CD key as long as you put the second CD (game CD) in the drive...
What you can try :
1. If you have several CD drives, try in both of them
2. Try shutting down programs which may interfere with the CD drive, like drive emulators, burning programs, media players...
3. If you have previously installed a "copy" of the game, it may have left some garbage in the registry, please contact
support@larian.com they'll tell you how to clean it up.
4. If nothing works, contact
support@larian.com anyway <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Btw, the technical FAQ may provide some more help for generic problems :
http://www.larian.com/riftrunner/FAQ.php?section=9 
