The other night I dreamt that people were speaking Dutch. Not odd in itself, except I'm not even semi-fluent in Dutch. How did I know it was Dutch? In my dream, I just knew. So, was my brain making up something that sounded like Dutch? Or was it repeating things I've heard from Dutch friends, and also from when I spent a few days in the Netherlands? (This would mean that the words were actually accurate Dutch, but probably incongruous to the dream.)
I occasionally dream in French, but the people I know that speak French only speak French if they show up in my dreams. Or if they are bilingual, I usually dream in the language in which I speak to them most often. However, in the dream I had where Dutch was being spoken, I didn't recognize any of the people.
But how about a language I have heard, but have never used?
In an article titled "In Your Dreams" by Stephen Dutch PhD (oddly enough), Dutch states that you don't necessarily need to be very fluent to dream in another language. He also writes that the subconcious is really good at recording information. In addition, your brain can know another language and still have difficulty retrieving it during your dreams. According to Dutch, dreams seem to be no more than "random memory dumps" (Dutch, 2010).