I came across this (on a great blog, by the way – http://www.thedailywh.at).
So, dreams seem to process what you’ve thought about during the day. So, I’d imagine that if you’ve been taking in languages during the day, your brain then kind of processes them during the night.
It’s just another reason for regularity of language learning.
One lesson per week, however good the lesson, just isn’t enough. Regular learning is the key, so that during the night, when you dream, your brain then SUBSCONSCIOUSLY processes the languages.
It’s a SUBCONSCIOUS process, not a CONSCIOUS one (and that’s true both for when you speak and when you understand, most of the time).