It's a good thing to speak French if you go to Corsica. English probably won't help you much; it is only spoken on camping sites, in hotels and tourist information offices, and by young people, but with about the same probability as German... If you speak Italian you're better off, since Corsica is visited by many Italians and Italian is quite similar to Corse and therefore spoken by many Corse, especially at the eastern coast.
But don't feel afraid to go to Corsica just because you don't speak French fluently. In case of an emergency you've got a left and a right communicator, and the school lessons pop up when you're there for some days.
Thomas is the one of us who speaks French best (but far from fluently), and Markus and I speak a bit of French, too. Speaking English didn't help me a great deal, but having forced myself to having some looks at my old French books helped a lot.