The website is internationalized! About time too really. OpenStreetMap is a truly global project with amazing involvement from people all over the world, but now the website can start to speak many languages accordingly!
How to translate
We're using translatewiki.net for our translations. Just go there and create an account and then start translating the OpenStreetMap website into your language. You can also view the current translation statistics there.
Text files in YAML-format are checked into The Rails Port git repo under config/locales. People can provide translations for the various bits of the website. See Committing to the rails port. You can browse the yaml files here
More recently Shaun McDonald figured out main i18n code now in use. This was based partly on Arindam Ghosh's work, and taking into account some new internationalization features of Ruby on Rails.
The website finally went multi-lingual after much hacking at the London Hack Weekend May 2009, with various people working on converting rails views, and the usual suspects (Tom Hughes) debugging and deploying. blog post
Ævar Arnfjörð Bjarmason has done a lot of work after the branch was merged on making the missing bits translatable. E.g. adding parameters instead of using message concatenation. He also worked on the merging to translatewiki.net.