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 300 localisations of MediaWiki and its extensions use translatewiki.net as a convenient translation medium, as well as for coordination and maintenance. Mantis, FreeCol and OpenStreetMap 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.