LANGUAGE -- language code for the current user
Returns the language code for the current user. This is the language used by Foswiki to generate the user interface.
The language is detected from the user's browser, unless some site/web/user/session-defined preference setting overrides it.
preference is explicitly set, this will be used as the user language instead of any language detected from the browser.
in a non- per-user way, otherwise users will not be able to choose their preferred language.