When you wonder in what languages MediaWiki has been localised, and to what extend the localisation is usable, BetaWiki has some great statistics.
The Localisation statics show the languages that have a central localisation and the percentage of the messages that have been done. It clearly shows that the MediaWiki localisation leaves a lot to be desired; for some 144 languages less then half of the messages have been localised. At this moment there are 235 languages known to MediaWiki. When you compare this to the 253 language that have a Wikipedia and add the languages that are starting in the Incubator, you get a clear picture of how much effort is needed to better support the readers of MediaWiki projects.
When you look at the statistics, the glass is half full, and it is filling. On average five languages are introduced every month and more then 500 messages are translated every day. The languages that are in the Incubator are doing well Seeltersk for instance has done an astonishing 99,3%.
One of the latest innovations in the BetaWiki are the core top 500 messages, they contain the most important messages and with these messages translated, MediaWiki is usable for a language. BetaWiki has a dedicated team of people that make MediaWiki and as a consequence MediaWiki projects usable for many people. With your help, we can improve the localisation even further. One message at a time will slowly but surely provide proper support for all the languages MediaWiki supports.