My native language is Finnish. There are plenty of dialects in Finland, and the official written form of the language is kind of an artificial synthesis of various dialect traits -- nobody really speaks written Finnish.

That is, except me when I was six... My parents would go to a school meeting and the teachers and other parents would be surprised -- "Oh, you don't speak in written language like your son?".

I guess I had been spending quite a bit more time with books rather than people.