Could it happen that the people speak German, English and Slovak in one sentence in my dream? I think I would laugh if it happened xD
Oh yes that has happened to me. A dream character once spoke in japanese, english, and french in one sentence. But it's all the language I knew though. It was broken, but it made sense.

How can someone's brain use a language which someone doesn't speak? Or could you already speak Mandarin at that time?
It was around when I was 11 and I did not know mandarin at the time. In fact, right now even as an adult I only know few phrases. I'd like to think the dream was tapping into a collective consciousness of sorts, but it could be something I may have heard on TV or online. ^^'