Hi @ygern!
I noticed one interesting thing in your post.
you wrote "my mother tongue" and I even start to thing that you're Russian because Russian language does use word "tongue" as a word for language.
We don't have 2 separate worlds : tongue for part of body and Language as a method for communication. Instead if it Russians use word "tongue" in both cases
Hi. It's interesting. :D
I just check the dictionary. lol.
and found that the word "tongue" in English do carry the meaning of "language."
While substitute "language" with "tongue" is not common, using "mother tongue" to refer to native language is actually quite a common usage.