Wikipedia, "First language" ([Internet, 2025-01-27](https://en.wikipedia.org/wiki/First_language))
Children growing up in bilingual homes can [...] have more than one mother tongue or native language.
EXP: in the context of population censuses conducted on the Canadian population, Statistics Canada defines mother tongue as "the first language learned at home in childhood and still understood by the individual at the time of the census"
after Wikipedia, "First language" ([Internet, 2025-01-27](https://en.wikipedia.org/wiki/First_language))