How will my child's language skills develop if he's learning two languages?

It's interesting to see how language works in bilingual families. If Mom speaks Spanish, say, and Dad speaks English, a child will answer his father in English and his mother in Spanish, even when those particular parents are speaking different languages than they normally do. Some bilingual kids mix the languages early on, and sometimes expressive language (what they say) is slightly delayed. But very quickly, those delays work themselves out, and when a child starts speaking in both languages, he will do so in full sentences that are contextually accurate and age appropriate.