Google Analytics language stats - where does this info originate?

accept-language or user agent, or other?

[badbadmonkey]

Google Analytics language statistics - where do these figures come from?

The first ranked language choice, i.e. accept-language, or the user agent, or maybe the system (OS) settings?

[ogletree]

This comes from the browser language setting

[badbadmonkey]

Which is what? The Accept-Language header?