Languages can start very young though. Locally I'm familiar with a lot of families of mixed-nationality. In my own case my partner's native language is Finnish and mine English; our child grew up hearing/understanding both, and now speaks both of them to the appropriate parent.

(It's quite fascinating to see that play out, we were walking in the street recently and somebody stopped to ask me a question. He butted in and said "Daddy speaks English", in Finnish, to the stranger.)