MediaWiki is used for projects in many languages. Wikipedia is the project best known for its many languages. The WMF Sitematrix gives the distribution of the languages and its projects. When you consider the usability of the MediaWiki software, you would expect that there is a correlation with the size of the project.
When you consider the top 10 Wikipedias, nine need localisation and the localisation for German, French, Swedish, Japanese, Dutch and Portuguese are fine. Italian and Polish need work done on the extenstions as used by the WMF while Spanish needs work done on both the MediaWiki messages and the extensions.
I am really happy how the quality of the Japanese localisation has improved. It is well covered for usage in the Wikimedia Foundation and we are awaiting the creation of a Wikiversity in Japanese.
Italians, Polish and particularly Spanish are the odd ones out. I am amazed that the communities for these languages have not picked up the need for proper localisation. The Japanese example has shown that it takes only a few good men to make a big difference.
When you look at other languages like Hindi that have so many speakers, you really wonder why it is doing so poorly certainly when you compare it with other Indian languages like Bengali, Marathi, Telugu, Malayam, Bengali and Tamil.
When you are part of a language community, check out the statistics for your language. Help us at Betawiki to support your language; there is plenty of room for improvement in most languages.