This is a very interesting topic. Now that I think about it, I don't think I've ever had a dream in which I was speaking, that I was speaking in another language other than my native English. From what I can remember of my dreams, there isn't rally any speaking in them as much as there are thoughts that don't really come across as such. Non-verbal communication if you will. Kind of like the fact that when you are walking in your dreams, your feet never touch the ground. It's hard to explain.