Translatewiki.net is a project where a community of localisers make the user interface available in their language, which we've been using for a while also for OSM. Originally, it was dedicated to the internationalisation and localisation of MediaWiki, the software powering Wikipedia in almost 300 languages. As a consequence, it has a diverse community of people helping out.
For OpenStreetmap, we have a project page where you can find all the pertinent data on the OSM project. At this time we localise both the website and potlatch, plus Waymarked Trails. More software can be added by requesting on the support page, see also Translating:New project.
We are waiting with anticipation the realisation of the integration of OSM in Wikimedia projects. We hope that this will bring new people to both our projects. We are really happy that the OSM localisation people are joining translatewiki.net because as the functionality will be shared, we will become more relevant and better understood with a properly localised user interface.
Wikipedia uses the alpha code of the MediaWiki software and as a consequence, translatewiki.net is running the bleeding edge version of MediaWiki. It is because of this that people localise the latest available software and consequently, when software goes live, the localisations are included. In order to stimulate our community even more, we have developed an extension for MediaWiki called "LocalisationUpdate", it will provision the localised messages that fit with the software that is run. Consequently, the localisation of Wikipedia is as good as we can make it with volunteers.