On the user's first visit, default language from HTTP_ACCEPT_LANGUAGE

On the very first visit when no language preference cookie is set,
attempt to guess what the user wants from their HTTP_ACCEPT_LANGUAGE
setting from their browser. (Their OS language)
This will attempt to match the preferences in the order they're
specified by the browser, which should be the user's order of
preference.
For full language + country code preferences (eg en_US) it
will attempt to match the whole thing. For just language preferences
(eg en) it will match the first locale with that same language code.