When you're dreaming, you seem to speak the language as you think it and not as you really speak it, so if you can "think" in Spanish pretty well, chances are that you can "handle" a conversation in Spanish when you're dreaming.

I'm a native Mexican, and I speak Spanish (because I live in Mexico), but in most of my dreams I speak English very fluently, my dream journal is in English actually, even though in reality I don't speak English really well, it's kind of weird.