Don't worry - all cast members will speak good English. A little bit of French will never go wrong, bonjour, merci, s'il vous plait etc just to show courtesy.
As for the shows, some have versions in different languages (Stitch, Armageddon, off the top of my head)
Others have headsets where you can choose your language, and last year the main Mickey's Magical Party Show repeated the lines in French, English, German and I think Spanish as well. I'd imagine this year's show will be similar.
I think Disney have done a great job of dealing with the obvious language problem posed by being in Europe. I have never felt at a disadvantage due to my rubbish grasp of French!