I have a BA degree in English Language and Culture, so I do have a basic linguistic background and I know what you mean, Lucho.
I’ve recorded lots and lots of dreams and I usually remember very many details when I’m into it. I can only speak from personal experience, though.
My dreams are usually a mixture of languages. For instance, the two languages I use the most are English and Dutch. I know this because I often remember specific conversations I have with people that are either in English or Dutch. Sometimes bits and pieces will be in other languages that I know a little of (German, Swedish, French and Italian mainly), but in those cases I do think my mind makes up for my lack of knowledge and fills in the gaps with “dream language.”
I’ve never actually remembered any specific conversation in any other language than Dutch or English. This is because those are the only two languages I speak 100% fluently. And I use both a lot in daily life.
What language of the two features in my dreams is heavily dependent on several factors. Firstly, the other people involved. If any of my family members are involved, they will simply speak Dutch, as they do in real life. My best friend speaks English, like we do IRL. Most of my other friends also speak Dutch in my dreams.
However, I often dream with lots of DC’s that my subconscious made up. That almost always happens in English. This is likely because English is the dominant language in my life. Everything I do for myself, I do in English. So whenever I’m “by myself” in a dream, English will be the default language, even though I live in the Netherlands in waking life.
Therefore, I would assume that the languages we use in our daily lives are carried over into the dream world to a large extent. When I started learning Swedish, I had a few dreams where at least some of the characters spoke what I interpreted as Swedish, but like I said, with only basic proficiency in Swedish, my mind had to fill in the gaps and basically make up the language.
As for Universal Language, I think there is enough scientific evidence to support the idea of Universal Grammar. Why is this important?
Let’s take French as an example. I learned this language in high school (alongside English and German), but I’ve forgotten most of it now. Yet the basics are still somewhere in my subconscious. The sounds of the language, certain grammatical features etc. It’s not a stretch to think that my mind would just tie certain elements together with Universal Grammar and “create” a language that in my mind sounds exactly like French. This is how we can dream in languages we don’t even speak. Our brain takes what we know, and uses the principle of Universal Grammar to construct something we can interpret more easily.
But I do believe that my English and Dutch dreams are in the proper language, because I can often recall specific conversations and I’ll remember exactly what I said in the dream, and what a dream character said. Whereas if I’ve dreamed in Swedish or French, I’ll only remember that I spoke (or heard) the language. I might also recall what was said (by that I mean the “basic meaning” of the conversation), but I’ve never actually remembered anything specific. This is because I’m not fluent enough in those languages.
So yeah, sorry for the long and unorganized post. It was meant mainly for the OP, but I hope some others have been able to get something interesting out of it. I’ll return to this thread later to see if anything else came from my contribution.
Definitely a very interesting subject.