Issue #11035 - the framework should distinguish locales fully supported (languages for which all translation files etc exist) from basically supported locales (datetime / numbers conversion rules and so on, known for virtually all languages - from CLDR)
The resources localization by adding locale-tag suffix (like foo.en-GB.png) can be problematic (it makes mess in directory) - it would be nice to have something like resource bundle in Java
Support for more data from CLDR (most important: sorting rules)
Internationalization of models (to do multi-language articles etc)?
These are the most important shortages AFAIR, if I find more I will post here :-).