Returns the language code for the language used as the current user. This is the language actually used by TWiki Internationalization (e.g. in user interface).
The language is detected from the user's browser, unless some site/web/user/session-defined setting overrides it:
If the LANGUAGE preference is set, it's used as user's language instead of any language detected from the browser.
Avoid defining LANGUAGE at a non per-user way, so each user can choose his/her preferred language.