lang (as an attribute name)

denotes an attribute whose value is a language code for the natural language of the content of any element; its value is inherited. This name is reserved by virtue of its definition in the XML specification.

Notes

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 BCP 47 at http://www.rfc-editor.org/rfc/bcp/bcp47.txt and the IANA language subtag registry at http://www.iana.org/assignments/language-subtag-registry for further information.

The union allows for the 'un-declaration' of xml:lang with the empty string.