Chomsky on kids learning languages...
https://www.youtube.com/watch?time_continue=12&v=eI4rNw6d7UY
Both my kids went to French Immersion and are fully bi-lingual as adults, although neither their dad or I speak French.
My son uses his occasionally, but my daughter landed a sweet government job because of her being bi-lingual and makes a bit more a hour there for having it (she uses it every day).
Both kids say that having the 2 languages helps them. On holiday in Spanish-speaking countries, they can usually understand what's being said because of the language similarities.
At the time, a lot of parents were afraid of French Immersion because most kids by Grade 3 or 4 are speaking/reading/writing a bit better in French than in English and parents panic. But it evens out eventually. Both my kids have said they are glad we sent them to French Immersion.
Are you considering it for your child/children?