http://www.w3.org/1999/xhtml
attributeGroup i18n

Documentation

      internationalization attributes
      lang        language code (backwards compatible)
      xml:lang    language code (as per XML 1.0 spec)
      dir         direction for weak/neutral text
      

Source

<xs:attributeGroup name="i18n">
  <xs:attribute name="lang" type="LanguageCode"/>
  <xs:attribute ref="xml:lang"/>
  <xs:attribute name="dir">
    <xs:simpleType>
      <xs:restriction base="xs:token">
        <xs:enumeration value="ltr"/>
        <xs:enumeration value="rtl"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:attribute>
</xs:attributeGroup>