Like many applications, MediaWiki has both core and extended functionality. There are a growing number of extensions and most are localised at Betawiki. For the developers who develop MediaWiki, Betawiki has proven itself a precious resource. Many of the MediaWiki extensions have been adapted in order to get proper internationalisation. Recently, an extension developed by the UNICEF was worked on for more then a day and necessary feedback was provided about the code.. It just did not work for people wanting to implement it.
As time goes on, extensions are localised in more languages. Siebrand has a list that every so often is updated with the extensions with localised messages in at least 50 languages. It is a list that is growing.
As can be expected it is the extensions used by the Wikimedia Foundation that are doing particularly well, the support for FlaggedRevs is something that amazes me because it is only used on two projects as far as I know. I am happy to see the Babel extension localised in 122 languages. This makes it a really useful tool, it is sad that the WMF does not use it because many of its projects would benefit from the big support it provides. The Meta discussion is pretty much in favour and it can benefit all our projects.
With Siebrand in a statistics mode, he also provided me with some more numbers.. Currently Betawiki supports 128 extensions with localisations in over 50 languages, in mid May there were only 64. In January, the extensions used by the WMF had 570 messages, currently there are over 1300... In January MediaWiki contained 1732 messages, now there are 2122, a growth of 23%... In January we supported 2077 extension messages, by now there are 5111, a growth of 246%.
It is all the more amazing that the MediaWiki localisation community does this well.