From MozillaZine Knowledge Base
The current locale in use by Mozilla is stored in this preference, and a variety of components consult it for localization information. As its name suggests, it is also used in constructing the user agent string sent with HTTP requests.
Possible values and their effects
A string, usually an ISO 639-2 value, corresponding to the user’s current locale. Default value varies by localization.
First checked in
Has an effect in