The fact that ImpressCMS is multi-language from its very core is one of the points of which we are quite proud. Recently, thanks to ep98 we started adding our translations to Transifex.
Transifex is a very capable service, but due to all the possibilites and functionalities it can become difficult to find your way.
A short introduction to translating with Transifex is now available on the wiki. Feel free to update or extend where needed, that's where wiki's are for