Maybe they just guess what he's saying and by some miracle always get it right? I don't know, I've never had any problem understanding what the heck Donald is saying (In the cutscenes at least), so I would only assume that Sora and co. do as well. Also, isn't it a little weird that Donald is the only duck that talks like that? Scrooge, Daisy and Huey, Duey and Louie talk just fine, so what's up with him? Eh, whatever, character quirks.