Mothertongue, first language, native language or dominant language?

In the strictest sense, we all have a mothertongue as we all have only one (biological) mother. – But does this mean that the language our mother did talk to us is automatically our mother tongue? What about this friend I had in school, who was adopted when she was 2 and grew up in a Dutch family: would her mother tongue be Swahili because her mum was talking Swahili to her or would it be Dutch, because this was the language the mother who adopted her did talk to her?

Usually, mothertongue defines the first language we were exposed to, our L1, the first language we speak, the one we grew up with or that our parents (or caregivers) did speak with us. And usually people tend to speak this language for a long time.

It doesn’t have to be the language of our mother – or father –…