Children raised bilingual or multilingual in the early days will mix languages. They make errors by using the syntax of one language and the words of another. (“Touch the guitar”, my old Spanish teacher’s daughter would say, instead of “Play the guitar”.) But these problems disappear quickly. By three or four, children reliably separate the languages, knowing which can be spoken with whom. Their fluency in each would be the envy of any adult language-learner. Many parents once believed that a second…