Another reference about languages from another book: The Collapse of Parenting by Leonard Sax.
He writes that childhood and adolescence are about enculturation. That’s when you “acquire” the skills and knowledge that you need to operate in the culture you’re in. That’s when you master its customs and behaviors. He says that it takes years to master a culture’s the details of any culture. Moving to Japan or Switzerland, one would most likely always feel like an outsider “even if we did manage to master at least the language after 20 years. The adult brain is much harder to alter.”