Q&yourA: Bilingual babies

Kjersti writes:

"Can any of your readers talk about raising children bilingual? I'm American but my husband is not. His mother is about to move in with us to help take care of our 4-month-old son when I go back to work in two months (I know this is going to be a whole new set of issues for another email). She'll speak to our son in her language (my husband already speaks that language to our son). I'll continue speaking to him in English.

I know his language development is likely to be a little slower than for babies who only hear one language. I guess I'm just looking for some kind of reassurance that he will eventually become fluent in both."

I do have to turn this one over to you, readers. All I know about bilingual language development is what I've read and seen. All three of my cousins were raised bilingual, and as adults they have varied preferences for first language, but all are fluent in both, and the kids in our playgroups who are being raised speaking both seem to be doing fine in both.

But I know there have to be plenty of you out there who can give better ideas of timelines and things to watch out for.

Can anyone help Kjersti with what to expect?