Hello! MODS 3.4 XML Schema available at http://www.loc.gov/standards/mods/mods.xsd defines xs:simpleType name="languageAuthorityAttributeDefinition" as an xs:restriction base="xs:string" containing (among others) xs:enumeration value="rfc4646" But RFC-4646 is now obsoleted and replaced by http://tools.ietf.org/rfc/rfc5646.xml which takes into account RFC-5645 containing some important improvements. Therefore, I would suggest to slightly modify http://www.loc.gov/standards/mods/mods.xsd to support RFC-5646, which could be achieved with the additon of xs:enumeration value="rfc5646" in the definition of xs:simpleType name="languageAuthorityAttributeDefinition" Regards! SaaĊĦha,