The abbreviation of the official language is entered in this element. The basis is ISO 639-1
for the language identifier with two lower case letters [1].
For more details on its usage, see the page on handling multiple languages.
<xs:simpleType name="stringISO639Type">
<xs:restriction base="xs:string">
<xs:pattern value="[a-z]{2}"/>
</xs:restriction>
</xs:simpleType>
The element of type stringISO639Type is configured in such a way that it can hold exactly two lowercase letters.