i18n 

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

Attribute Group Information

Attributes

QName Type Fixed Default Use Inheritable Annotation
dir restriction of xs:token optional
lang LanguageCode optional
xml:lang union of(xs:language, restriction of xs:string) optional 
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.

Used By

Attribute Group attrs
Elements font, head, html, isindex, map, meta, style, title

Source 

<xs:attributeGroup name="i18n">
  <xs:annotation>
    <xs:documentation>internationalization attributes lang language code (backwards compatible) xml:lang language code (as per XML 1.0 spec) dir direction for weak/neutral text</xs:documentation>
  </xs:annotation>
  <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>