lang

Attribute Group Information

Attributes

QName Type Fixed Default Use Inheritable Annotation
xml:lang union of(xs:language, restriction of xs:string) required 
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
         RFC 3066 at http://www.ietf.org/rfc/rfc3066.txt and the IANA registry
         at http://www.iana.org/assignments/lang-tag-apps.htm for
         further information.

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

Source 

<xs:attributeGroup name="lang">
  <xs:attribute ref="xml:lang" use="required"/>
</xs:attributeGroup>