You don't need to know French to play a Beauxbatons character. Making occasional grammar mistakes (as opposed to misspelling, as Becca said above) helps, but even that is not important. If your character wards something to a fellow Beauxbatons student it's logical that they'll talk to each other in French (or their native language if, for instance, they both happen to be Italian or from another nationality) but you can still write those messages in perfect English - all you need is to tag those wards as "Private to [name] (in French)" or something like that. In fact, it helps a lot if you do things that way, since other players will want to follow the conversation and they won't necessarily speak French either.