[Nyororin]

I too have to wonder the same thing. As long as one side doesn`t have Japanese citizenship, the child is listed as having a foreign parent. This will include the Chinese and Korean populations of Japan - most of which were born in Japan, have never been outside of Japan, and only speak Japanese. Then again, with how few babies are being born, it`s certainly not completely unthinkable.
Although I have a feeling that most of the babies were born to Asian + Japanese couples, and will look entirely Japanese. Those who aren`t, I`m guessing have a pretty high chance of moving out of Japan... As it seems like that`s the popular thing to do if the foreign side is from anywhere other than Asia. Even more so, obviously, if both parents are from outside Japan.