When you localise software, the information that is provided in a message may differ. There can be none, one or more instances involved of what the messages informs about. Depending on the numbers involved, the sentence differs and support for singular and plural is needed.
MediaWiki supports this. However, it is not obvious how plural support should work as it differs from language to language. Welsh for instance has more then five forms of plural to consider. When you want your language to support plurals, it needs to be clearly defined how this should work and once this is done, you either post a bug to Bugzilla or ask the fine people at Betawiki to set it up for you.
Nikerabbit has recently implemented the plural support for Lithuanian. As a reaction a bug was entered for Russian with a request for plural support. On the Betawiki tasks list there was a comment that there are many more languages that could profit... Hr, Cu, Cs, Be, Be_tarask, Sk, Sl, Sr_el, Sr_ec ...
When plural support has been implemented, it means that a lot of messages have to be revisited to create the right text. These messages can be found on the "List of warnings" where messages with problems are indicated.
Supporting plurals happens in two phases; first the software is adapted for a language and then all the messages with plurals have to be adapted. It is a lot of work but it sure improves the user experience :)