Why does Safari think I read Japanese?

When Safari requests a Web page, it nicely uses your language preferences that you set in System Preferences, which is what you would expect. Here's something strange that I stumbled upon — for some reason, Safari always adds Japanese to the list of languages you accept (in the "Accept-Language" header), whether it is on your list of languages or not. Why?

I'm not the only one to notice this. In any case, I've reported it as a bug.