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:element name="conceptItem"><xs:annotation><xs:documentation>An Item containing information about a concept.</xs:documentation></xs:annotation><xs:complexType><xs:complexContent><xs:extension base="AnyItemType"><xs:sequence><xs:element name="contentMeta" type="ContentMetadataType" minOccurs="0"><xs:annotation><xs:documentation>Content Metadata for a Concept Item</xs:documentation></xs:annotation></xs:element><xs:element ref="concept" minOccurs="0"/></xs:sequence></xs:extension></xs:complexContent></xs:complexType></xs:element>
ANY attribute from ANY namespace OTHER than 'http://iptc.org/std/nar/2006-10-01/'
Source
<xs:element name="contentMeta" type="ContentMetadataType" minOccurs="0"><xs:annotation><xs:documentation>Content Metadata for a Concept Item</xs:documentation></xs:annotation></xs:element>