I've had this happen, too. I assume you must have a certain level of fluency in the language before you can do this. For example, my Swahili is only passable, but my French is excellent. I have French dreams all the time, but almost never Swahili. I've tried speaking both in lucid dreams. I amaze myself by being a pro at either one when I'm dreaming. I fear I still may be fooling myself, because I can never remember the details after I wake. I think I might just be faking most of it.