And would it be easier than trying to do it while awake? The reason why I’m asking this is becasue I spoke arabic in one of my dreams, and I wondered if I could actually learn (or create) a language in my dreams.
No. I don’t see how it would be possible, as you would need to have the knowledge stored somewhere in your subconscious. I think re-learning a language you used to know could be plausible, as you’ve already learned it, but learning a language from scratch just doesn’t seem possible.
As for creating a language, yes, that could be possible, as you’re relying on your creativity more than real-world knowledge. However, you could also just make a language IWL…
Thanks!
Agree with fennecgirl. Creating languages is possible, although probably complex. You would need to think not only about the words, but also about the grammar, syntax, all the forms/tenses etc. Creating languages in WL, however, is a cool hobby. There is a nice guide on how to do that yourself: zompist.com/kit.html
Many people have become pretty successful with this. Ever heard of J. R. R. Tolkien, that has created more than 20 languages and applied them to the tales he has written? 
High five for the LCK! That’s my main resource for language creation. 
It could be done. Don’t know whether it would be easier/harder then in waking life. For some creativity is very high while dreaming. But for others logic or reasoning might be lower while dreaming. I think the hardest part will be remembeing what you created after waking up.
My suggestion is try it
my english isn’t good
Neither is mine, yet I’m speaking in it
you can do all that imagines you in dream including inventing of the languages 
hahaaha to say that french forum is to block