An anecdote to counter yours: I am French, my wife is Japanese and we live in Barcelona in Spain where the school is in catalan. Our four years old understands Japanese and Catalan quite well (similar to a monolingual kid) and can speak French, English and Spanish a bit (I speak to him in French, adn with my wife we speak English) but not as well as a monolingual kid. Ofmcourse we have to see how it will turn out for him, but to me in terms of languages he's pretty much like a sponge.