The language data type is derived from the token type : language « XML Schema « XML Tutorial

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<!-- schema -->
<xsd:simpleType>
  <xsd:restriction base="xsd:token">
   <xsd:pattern value="([a-zA-Z]{2}|[iI]-[a-zA-Z]+|[xX]-
   [a-zA-Z]{1,8})(-[a-zA-Z]{1,8})*"/>
  </xsd:restriction>
</xsd:simpleType>

Restrict language type using enumeration.

<!-- schema -->
<xsd:element name="language_native">
  <xsd:simpleType>
   <xsd:restriction base="xsd:language">
    <xsd:enumeration value="de"/>
    <xsd:enumeration value="fr"/>
    <xsd:enumeration value="en-GB"/>
    <xsd:enumeration value="en-US"/>
   </xsd:restriction>
  </xsd:simpleType>
</xsd:element>

<!-- instance document -->
<language_native>fr</language_native>