The W3C Internationalization Core Working Group has posted the first public working draft of Language and Locale Identifiers for the World Wide Web (LTLI). This draft doesn't seem to say very much except that languages and locales are not the same thing, and
Specifications that make use of language or locale values MUST meet the conformance criteria defined for "well-formed" processors, as defined in sec. 2.2.9 of [RFC 3066bis].
Specifications that make use of language or locale values MAY validate these values. If they do so, they MUST meet the conformance criteria defined for "validating" processors, as defined in sec. 2.2.9 of [RFC 3066bis].
Specifications that define operations on language or locale values using matching Must use either a basic language range or an extended language range.
Specifications that define operations on language or locale values using matching MUST specify whether the resulting language priority list contains a single result (lookup as defined in [RFC 3066bis Matching]), or a possible empty set of results (filtering as defined in [RFC 3066bis Matching]).
Specifications that describe the identification of locales or aspects thereof with IRIs may use IRIs [RFC 3987] for this purpose, or to point to more detailed locale or preference data.
