There are several people on the web to whom a phrase like this is attributed:
„You can speak 20 languages – but if you cut yourself you speak your mother tongue!“
Of course, that’s true. The mother tongue is the main language of a person, his or her language of the heart. This will not change in life even if you marry a person who does not speak your mother tongue.
You may get fluent in another language, or even several languages; you may move to another country where your mother tongue is not known and not learned; yet your mother tongue stays the same.
I found out that there is one exception,
in which teenagers seem to switch their main language from the mother tongue to another one: when they had several years of schooling in a different one.
They spend 80% of the day in a different language and have to do homework, to deal with their friends and to do many hobbies in a different language. When the home culture of the parents is not your own – then it might happen.
One example: We met a young guy with an American passport in Southern France who had spent most of his life in French-speaking Belgium. Our group of friends was mostly English or German mother tongue speakers and he was asked quite soon to translate from French into English. No chance: he failed completely. No word came out of his mouth.
Two weeks later, someone asked him the other direction: from English into French. No problem, he did it without thinking.
French was his main language, his „mother“ tongue.