I know both English and Portuguese fluently but since I live in the UK I predominantly have English language as the basis of my thoughts, dreams etc. So I've had a few experiences in dream, both lucid and non-lucid where I decided to speak Portuguese or a DC decided to speak in Portuguese. It's always surprising either way. So a couple of times I spoke in a lucid and non-lucids it seemed like I caught some DC's of guard as they didn't understand what I said and even after repeating I had to result to speaking in English to find the DC responding. Truly fascinating stuff. There was a time when one Dc understood but the other didn't as well.

Then there was one time when I was speaking to a DC friend of mine which speaks English in waking life and she spoke to me in Portuguese, but just that once. That really took me by surprise. It was sort of a revelatory message I needed to hear, which was very clear, unlike most of the time when we need to figure out the meaning of certain symbols.

Don't know anything about how it would be when you are learning a language but I did have one dream once where a DC said a word in Japanese which I am pretty sure was right. I mean I don't know it at all but it's one of those words that are common enough, but not so often that you hear all the time. Think he said "Thank you" in Japanese and then I found out how that sounded and it seemed to be the same word.It's funny because in dreams we don't even need language and can just know people's intent and meaning.