Yes, I have. Although I'm not sure why. It might be because I am good in English, but I doubt it. Maybe once or twice though, because I had absolutely no reason to not talk Dutch in my dreams.
It happens quite often, but I guess that's because I think in English a lot. And then you get used to the language, so it becomes easier for your subconscious self to 'speak it', probably. I'm almost sure dreaming in English doesn't always mean you can speak it perfectly, because I once used a word I didn't even know. Well, I'd heard of it, but when I woke up I had to look it up in a dictionary because I didn't know what it meant ^^. I think my brains just remembered the word for some reason and used it for lack of a better word.