The W3C Web Services Internationalization Task Force has published the first public Working Draft of Requirements for the Internationalization of Web Services. According to the intro,
A Web Service is a software application identified by a URI [RFC2396], whose interfaces and binding are capable of being defined, described and discovered by XML artifacts, and which supports direct interactions with other software applications using XML based messages via Internet-based protocols. The full range of application functionality can be exposed in a Web service.
The W3C Internationalization Working Group, Web Services Task Force, was chartered to examine Web Services for internationalization issues. The result of this work is the Web Services Internationalization Usage Scenarios document [WSUS].Some of the scenarios in this document demonstrate that to achieve worldwide usability, internationalization options must be exposed in a consistent way in the definitions, descriptions, messages, and discovery mechanisms that make up Web services.
Mostly this document proposes new SOAP features for expressing various user preferences for language, number sorting, date formats, etc. However, it also suggests the possibility of extending language tags (en, en-US, en-US-LA, en-US-LA-YAT) with other locale information such as time zone and collation preferences. At first read, that doesn't sound like a very good idea to me. I think these ought to be separate, independently parseable items rather than tying them all together in one unmarked-up string.