This sounds so familiar to me! My partner tells me that I talk in my sleep on a regular basis. I am multi lingual so speaking other languages than English is not unusual for me. Except for the fact that I once started to speak Russian in my sleep! I have never been taught Russian!
The only words I know in Russian are Da and Njet...oh and Vodka
But speaking fluent Russian? No way!
A simmilar thing happened in a very clear dream. A lady in my dream spoke a different language to me, and I could not tell what it was. She yelled:" Maktub, Maktub!" at me. And I did not understand what "Maktub" was...and I did not know what kind of language it was... until years later I talked to a Turkish friend about this strange dream.
He told me "Maktub" is Arabic, and it can mean three things depending on the context.
"the letter" , " It has been written" or " It is your destiny"
In Arabic-speaking countries it is a common expression that your destiny is written on your forehead.
What puzzled me, was that up until then...I had never learned any Arabic words!
So how could I dream something I had never learned before... if dreams were only compilations of our memory banks?
I have, as far as I know, only spoken in terrestrial languages subconciously but it was enough to get me intrigued.