3-character language tags are specified in RFC 3066
("http://www.ietf.org/rfc/rfc3066.txt"). This document gives also an advice
on how to proceed in when different tags are applicable:

>2.3 Choice of language tag
>
>    One may occasionally be faced with several possible tags for the same
>    body of text.
>
>    Interoperability is best served if all users send the same tag, and
>    use the same tag for the same language for all documents.  If an
>    application has requirements that make the rules here inapplicable,
>    the application protocol specification MUST specify how the procedure
>    varies from the one given here.
>
>    The text below is based on the set of tags known to the tagging
>    entity.
>
>    1. Use the most precise tagging known to the sender that can be
>       ascertained and is useful within the application context.
>
>    2. When a language has both an ISO 639-1 2-character code and an ISO
>       639-2 3-character code, you MUST use the tag derived from the ISO
>       639-1 2-character code.
>
>    3. When a language has no ISO 639-1 2-character code, and the ISO
>       639-2/T (Terminology) code and the ISO 639-2/B (Bibliographic)
>       code differ, you MUST use the Terminology code.  NOTE: At present,
>       all languages for which there is a difference have 2-character
>       codes, and the displeasure of developers about the existence of 2
>       code sets has been adequately communicated to ISO.  So this
>       situation will hopefully not arise.
>
>    4. When a language has both an IANA-registered tag (i-something) and
>       a tag derived from an ISO registered code, you MUST use the ISO
>       tag.  NOTE: When such a situation is discovered, the IANA-
>       registered tag SHOULD be deprecated as soon as possible.
>
>    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.
>
>    6. You SHOULD NOT use the MUL (Multiple) tag if the protocol allows
>       you to use multiple languages, as is the case for the Content-
>       Language:  header.


Dieter Köhler