I checked right now and it shows that they're both full for me so I can't try adding them like you did.
You still have some time to drop your other course without it being permanently charged to you so don't drop it yet. I would e-mail the prof or the head of the language department (I think french would be under that department) and ask them to check about your registration in that course. Say that you've been having problems with it and you want to be sure before you drop another course. My biology and psych undergraduate advisors get back to me pretty quick so it shouldn't be too long of a wait to find out.
Edit: Right well I didn't notice how fast time flew. You do only have until Monday to drop that course... so if you've gotten French and it shows up on your timetable I'd print screen that and your course selection list just to be safe and then drop the other course so you have SOME proof that you added it in case the system screwed you over but PTGregD below did bring up the good point that it is a very sought after course from what I've heard so yeah, I guess you got dam lucky