Studies have disproven the old folk wisdom that a multilingual child is only a fraction as good at each of the languages it speaks—the different languages coexist in the brain and don’t take away from one another. In other words, it’s not like you’ve got a maximum capacity for a vocabulary of thirty thousand words that needs to be shared among the three or four languages you speak; each language gets its own vocabulary storage space in the brain, and no one has yet found a limit.
