Last modified: 2014-07-19 08:51:11 UTC
Tracking bug for all issues related to plural rules support. Should mostly be implementation in specific messages nowadays, and it's the responsibility of the maintainers of the specific piece of code or of volunteers in these cases (as far as I know).
Compare: bug 30443.
Note that many many more requests are among the <https://translatewiki.net/wiki/Support/Open_requests>
In case you don't find clear instructions, this old revision has more examples of how to add plural support: https://www.mediawiki.org/w/index.php?title=Manual:Messages_API&oldid=645340#Add_PLURAL_support