translatewiki.net is a localisation platform for translation communities, language communities, and open source projects. It started out with localisation for MediaWiki. Later support for MediaWiki extensions, FreeCol and other open source projects was added.
Over 180 localisations of MediaWiki and its extensions use translatewiki.net as a convenient translation medium, as well as for coordination and maintenance. Mantis, FreeCol and Mwlib.rl can also be translated in translatewiki.net. More projects are welcome.
You can participate in the translation effort into a language you know. The translations of the MediaWiki interface into new languages for requested projects of the Wikimedia Foundation are also done there.
This project is not part of the Wikimedia Foundation projects, or any foundation or open source project. It is run by Nike and Siebrand, who both are MediaWiki developers and have considerable experience in i18n and L10n. The translation functionality is provided by a MediaWiki extension.