I can only suply with learning languages, tried a bit in practice with my own kids and it works out fine...
Learning by doing in real life, classrooms can come later...

I am Danish, the wife Thai, we live in Germany, and our 3 kids attend an International school in an English section...
Basicly when the kids grew up, I spoke Danish with them, the wife spoke Thai with them, we let them attend German kindergarten at age 3, so they had to speak German with their playmates, and then we let them start in school at age 6, with English as the main language , so there they had to speak English with their playmates.. In order to prepare them for English we only let them watch cartoons/kids movies in English....so by the age 6-7 they had 4 languages, no problem at all, now they are in their teens and had added some Latin, Spanish and French on their own...The main thing is, I have never forced them to do anything, let it all come naturally by the surroundings, playmates....sure in the beginning they mix the languages a bit, but after a while the learn by themself to seperate...
The moment that you force classes or books on kids, you have lost..let it come naturally