The study of language acquisition, how we learn languages, must be a fascinating pursuit. Among the many questions researchers are probing are questions about bilingual acquisition: How do babies realize they're in a bilingual environment? What clues aid them in discovering this? How is discrimination between languages produced in infants?
If you're interested in this topic you might go to this article at Science Daily where you'll find a number of links to stories that address research in language acquisition and how children in bilingual homes recognize that they're hearing two different languages.
Whatever researchers eventually discover about these questions it's important to remember that it's all a result of random mutations in our DNA that somehow produced this astonishing capability in our brains. Having read my Dawkins I am assured that if I put on a blindfold and randomly struck keys on the keyboard in front of me I could, if I did it long enough, wind up converting these very words you're reading into Longfellow's Midnight Ride of Paul Revere.
It's logically possible, no matter how improbable, that a series of accidental gene mutations just by chance produced the ability to learn multiple languages simultaneously. And since it's possible, the Darwinian logic goes, therefore it happened.
Now recite that to yourself as long as it takes to get yourself to believe it.RLC