Use of xml:lang in XML Schema

I want to require a particular element in an XML Schema to be required to
have an xml:lang attribute. I am not, however, sure a) whether or not I need
to declare a separate xml namespace b) what URL I should assign that
namespace c) whether or not there needs to be a formal declaration for the
attribute in another schema fragment that should be imported/included into
the referencing schema. Your advice on the correct procedure for this would
be appreciated

Martin Brayn