Very interesting to think about. It started me thinking that there are probably several ways to define the words so I checked Wiki and there was a very interesting article and quote, especially with regard to cases such as ours. "For many children whose home language differs from the language of the environment (the 'official' language), it is debatable which language is their "native language". Native language being synonomous to first language in this case.

Here is the article.
https://en.wikipedia.org/wiki/First_language