Babies should be stimulated in more than one language to keep neural connections alive.
Babies are born, as we said, with the neural connections in place to hear all of the building blocks of phonemes for all languages.
But if these connections are not stimulated they will die out.
Hence, it is important that families with multi-language backgrounds should make these resources available to the babies.
Family members speaking different languages should talk to the babies as much as possible.
Some folks play language tapes to the baby, but I am skeptical of this approach, since it is not accompanied by the sense of vision or touch, and may be tuned out.
We shall have to wait for some research in this area.
I don't recommend a multi-language approach for children who are at risk for a language delay, such as premature babies, blue babies or babies with a history of language delay in the family, etc.
NOTES: Here are some hints for increasing phoneme awareness.