My sister and her husband have taught English as a second language in Japan and Canada (to the Japanese) for over 5 years. Her two daughters (now 4 and 8) were both raised in the Japanese school system, so are both completely fluent. They learn their letters over there much earlier, so it meant that they were also reading japanese before english. So their Japanese was grammatically correct, and they were considered little blonde Japanese kids!!
Earlier this year they moved back to NZ to resettle, and the elder daughter had some troubles at first with her English - particularly reading and writing. Within about 4m she was the top of the class and placed into accelerated learning.
They still sometimes talk Japanese - to be honest they actually dont like it anymore - they would rather be the same as their friends.
My sister and BIL also speak Russian as they lived there for a year, but that is their "secret" language. They use it to talk about things they dont want the kids to understand!!!!
The funny thing is my sister failed languages miserably at school - it was only after uni she learnt Japanese and Russian!!