I've studied Russian and Spanish (with some small fluency, but not enough to be very talkative). I've never had a dream involving Spanish, but I've had a few where Russian is spoken.
With one, I held a very, very simple conversation with just a couple of sentences. With the second, I was annoyed by this group of people and spoke Russian to them to signal that I wasn't interested in talking to them. I couldn't think of a word or phrase and just made up something randomly and actually became lucid because of it, knowing I wouldn't pull random garbage out of the air if I didn't know the translation. I was rather amused when I woke up.
With the third, a woman started speaking Russian but I couldn't tell what she was saying. I'm thinking it's because of my fear that if someone finds out I can speak Russian (or Spanish) then they'll start talking and I will have no idea what they're saying. Oh well :)
So, it just depends on the situation and on the DCs. I don't believe in "dream laws" just for the sake that the term "law" implies something that is solid and unchanging. I lean more towards the term "tendency," but it's just a matter of semantics :)
