[...] 5. You SHOULD NOT use the UND (Undetermined) code unless the protocol in use forces you to give a value for the language tag, even if the language is unknown. Omitting the tag is preferred.
I talked to Harald, the author of the RFC (3066), and he says that he sees xml:lang="und" as reasonable; we don't see any better alternatives for signalling "this isn't in any human language (so shouldn't be spellchecked, etc), but I need to specify a language tag to keep the higher-element's language tag from applying here".
-- Sean M. Burke http://search.cpan.org/~sburke/
