Constraining xml:lang - Catch 22

I have been asked why I don't validate xml:lang in my schemas to restrict 
its values to something like en-GB, en-US, en-Ca, fr-CA, es-MX.  But if I 
roll my own or derive something by restriction in my namespace, it may be 
my:lang but it is no longer xml:lang.  But for so many 
internationalization/localization reasons everyone wants the instant 
recognition and standardization of xml:lang.  Apart from using Schematron or 
depending on application program logic, are there any other useful 
strategies in this area?

I love this, from "http://www.w3.org/2001/xml.xsd"

<xs:attribute name="lang" type="xs:language">
  <xs:annotation>
   <xs:documentation>In due course, we should install the relevant ISO 2- 
and 3-letter
         codes as the enumerated possible values . . .</xs:documentation>
  </xs:annotation>
</xs:attribute>

Define "In due course".

Cheers Jack

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