People normally assume that "mother tongue" and "native tongue" mean the same thing. Even dictionaries assume this.

I noticed this when somebody (a native of the country I currently live in for 32 years), asked me in the last millennium what my mother tongue was.

My answer was "my mother tongue is x, and my native tongue is y", to their puzzlement.

For people who are born bilingual, as is my case, their mother tongue can be different from their native tongue.

But now that many people move around the world for career (and other) reasons, particularly in academia, there are people who have spoken (and written) a language for more than 50% of their life that isn't their mother or their native tongue.

What should this tongue be called?