xml:lang how often used?

Hi,

It seems to me that people are always reinventing the wheel where
xml:lang is used and I am always seeing standards that specify an
attribute or element usage for specifying the language that parts of a
document are in. Can anyone think of any particular formats or perhaps
governmental initiatives where it has been officially decreed that
xml:lang must be used when appropriate, and does anyone know why
people seem to not use xml:lang in favor of using other ways to do the
same thing?

Cheers,
Bryan Rasmussen