Schooling in an extremely significant factor. By the teenage, a child has undergone more than ten thousands hours of schooling (say, 6h * 250d * 8y), in the language of the country they grow up in. It's an extensive and, by an adult learning a second language, irreplaceable experience.

Children who have one parent from a foreign country, even if they communicate extensively with such parent, will have a considerably more developed native language.