MediaWiki, the software that runs, among others Wikipedia, is localised in many languages. Most new projects get their localisation as part of the software, the only thing left after an installation is to customise the system messages where neede. When a language is truly new to MediaWiki, the localisation can be done locally or centrally. When the localisation is done centrally, the messages are merged almost daily into the MediaWiki core and become available in all Wikimedia Foundation projects within a few days and every four months in a new MediaWiki release.
So far all the new localisations happened as a result of a request for new Wikipedias. Yesterday I noticed that the localisation for the Tuvin language, was driven by a project that is not within the WMF. You then ask yourself if there are localisations for languages elsewhere. I asked Angela, and she pointed me to several constructed languages hosted by Wikia. Some of them, like Aeres and Anglish are so new, that the Wikia project is likely the first real effort to realise these languages. A private use code can be used to indicate the language used until there is sufficient maturity for recognition.
There are Wikipedias in over 250 languages. The quality of the localisation of MediaWiki however is still patchy. It has improved a lot with the great work of Niklas Laxström and because no new WMF project is started without a good localisation for the language.
With so many people involved, all volunteers, it is not easy to get and maintain the desired quality. This quality is however essential as MediaWiki is a tool that supports more languages then virtually any other application and has an installed base that is growing rapidly.