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: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>
Schema location
file:/C:/Program Files/Oxygen XML Editor 16/frameworks/xml/xml.xsd