I think it's possible to replicate Spanish in a dream, if you've heard it enough (correctly) in real life. As in, if you've seen movies or heard songs in Spanish, not just "I heard another native-English speaker speak it."

I'm unsure how dreaming will affect the syntax of the language. The accents sound really good, and the words are actually understandable, but in my non-lucid foreign dreams, the sentence structure can be wonky, and the DCs may use Spanglish (even if I know the Spanish word for what they are speaking in English!).