First off, I believe DLP is one of the best theme parks. I consider it a "Big Kid's Disney", even though there are plenty of things for younger kids. I'm not sure how much time you have, but if you had to, you could cut out Disney Studios. I've been twice to DLP, and I find 3 days to be all the time you need to hit all the rides plenty of times. Again, I'm not sure how much time you need.
As for the fastpasses, if I remember correctly, you can only get one at a time. The Big Thunder Mountain tickets always seem to go first, so try to hit the ride first thing in the morning and get fastpasses if you want to ride it again. Also, Space Mountain lines get pretty long quickly, so also try to hit the fastpass on that. During any parades, the rides clear out pretty well, so hit rides then (I'm pretty sure there are parades at 2:00 and 4:00).
Most rides at DLP have loops (Indiana Jones, Space Mountain, Rock 'n Roller Coaster), though there are rides that are calmer (BTM). Bear that in mind if you have a child or a weak stomach.
If you have time, do go to Disney Studios. The show, Moteur Action Stunt Show, is incredible. If you have time, see it. If not, the park gets really empty, so rides like Rock 'n Roller Coaster or the Flying Carpets (which I waited in line for 30 minutes) are really easy to ride 2 or 3 times. Also during the movie parade, lines get really short.
The Studio Tram Tour at Disney Studios is always interesting, featuring Catastrophe Canyon. If you have time, do that. The line is long, though, and it had no fastpass when I went, so hit it if you can.
The castle is beautiful (with a hidden surprise underneath), and all the CMs are extremely helpful. Almost everyone speaks at least 2 languages, one being English. So if your French isn't that great, you'll do fine. Hope you have a great trip.
~William~