xml:lang

<xs:attribute name="lang">
  <xs:annotation>
    <xs:documentation>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.</xs:documentation>
  </xs:annotation>
  <xs:simpleType>
    <xs:union memberTypes="xs:language">
      <xs:simpleType>
        <xs:restriction base="xs:string">
          <xs:enumeration value=""/>
        </xs:restriction>
      </xs:simpleType>
    </xs:union>
  </xs:simpleType>
</xs:attribute>