I am amazed everyday at how smart kids are and how easily they learn things.
I am English, but DH is Afrikaans, and our home language is Afrikaans. I had plans of speaking only English to our kids, but it was just easier to speak Afrikaans.
When Amy was 18 months old we came over to Taiwan. At this time she was only used to Afrikaans. We put her into a Chinese daycare. One of the teachers could speak English, but she didn't really know English so they only spoke Chinese to her.
2 years later she speaks Chinese so well, that we have out her into a Chinese preschool. We don't speak Chinese at all though, so her 1st language is still Afrikaans but her 2nd language is Chinese!!
We have a lot of English speaking friends so we also speak English quite a bit, I have actually started taking the time to teach her some English. She is too cute, she will come to me and say "Let's speak English now". I would have to say that at the moment English is her 3rd language.
She definitely knows the difference between the 3 languages, and does translate basic Chinese for us.