lang (as an attribute name)

       denotes an attribute whose value
       is a language code for the natural language of the content of
       any element; its value is inherited.  This name is reserved
       by virtue of its definition in the XML specification.

Notes

      Attempting to install the relevant ISO 2- and 3-letter
      codes as the enumerated possible values is probably never
      going to be a realistic possibility. 
    

      See BCP 47 at
       http://www.rfc-editor.org/rfc/bcp/bcp47.txt
      and the IANA language subtag registry at
     
       http://www.iana.org/assignments/language-subtag-registry
      for further information.
    

      The union allows for the 'un-declaration' of xml:lang with
      the empty string.