Comment 5 for bug 550222

LANGUAGE should be supported I'd say. It's a gettext feature and therefore petty much standard on Linux systems.
With current nsLocale etc. it's a bit of a pain to implement it apparently though.
For example LANGUAGE can be a list of colon seperated locale names and nsLocale has no data structure for LANGUAGE. I'm currently a bit into the code and will try if I can come up with a patch.