In general you should separate the "guess the missing parameters" problem from the "matching a list of locales I want vs. a list of locales I have" problem. They are different.
Guessing the missing parts
These are all tricky areas, and even (potentially) politically charged.
But with very few exceptions the rule is to select the "original country" of the language.
The exceptions are mostly based on population.
So fr-FR for fr, es-ES, etc.
Some exceptions: pt-BR instead of pt-PT, en-US instead of en-GB.
It is also commonly accepted (and required by the Chinese standards) that zh maps to zh-CN.
You might also have to look at the country to determine the script, or the other way around.
For instance az => az-AZ but az-Arab => az-Arab-IR, and az_IR => az_Arab_IR
Matching 'want' vs. 'have'
This involves matching a list of have vs. list of have languages.
Dealing with lists makes it harder. And the result should also be sorted in a smart way, if possible. (for instance if want = [ fr ro ] and have = [ en fr_CA fr_FR ro_RO ] then you probably want [ fr_FR fr_CA ro_RO ] as result.
There should be no match between language with different scripts. So zh-TW should not fallback to zh-CN, and mn-Mong should not fallback to mn-Cyrl.
Tricky areas: sr-Cyrl should not fallback sr-Latn in theory, but it might be understood by users. ro-Cyrl might fallback to ro-Latn, but not the other way around.
Some references
- RFC 4647 deals with language fallback (but is not very useful in this case, because it follows the "cut from the right" rule).
- ICU 4.2 and newer (draft in 4.0, I think) has
uloc_addLikelySubtags (and uloc_minimizeSubtags) in uloc.h. That implements http://www.unicode.org/reports/tr35/#Likely_Subtags
- Also in ICU
uloc.h there are uloc_acceptLanguageFromHTTP and uloc_acceptLanguage that deal with want vs have. But kind of useless as they are, because they take a UEnumeration* as input, and there is no public API to build a UEnumeration.
- There is some work on language matching going beyond the simple RFC 4647. See http://cldr.unicode.org/development/design-proposals/languagedistance
- Locale matching in ActionScript at http://code.google.com/p/as3localelib/
- The APIs in the new Flash Player 10.1
flash.globalization namespace do both tag guessing and language matching (http://help.adobe.com/en_US/FlashPlatform/beta/reference/actionscript/3/flash/globalization/package-detail.html). It works on TR-35 and can look beyond the @ and consider the operation. For instance, if have = [ ja ja@collation=radical ja@calendar=japanese ] and want = [ ja@calendar=japanese;collation=radical ] then the best match depends on the operation you want. For date formatting ja@calendar=japanese is the better match, but for collation you want ja@collation=radical