Could someone still get into the 10th Special Forces group with fluent knowledge of French?

10th group is pretty big, so it depends upon how you're trying to get in. If you're trying to be an SF guy in one of the line units, french isn't going to be a make or break to your entry; it'll help during the assessment of your packet, but little more. Now if you're trying to get into the group without actually being SF, your odds go up, depending upon your MOS. Even if you're fluent, they're going to push you through their language school anyway.