Before Version 2.1, translating of the OpenStreetBrowser user interface was possible on this wiki page(s). As this was easy for starters there are several drawbacks of this method, e.g. the long response times on this huge pages on load/save as well the manual action which was necessary to import the changes and update the wiki pages to the newest version.
Starting with Version 2.1 (currently available on http://dev.openstreetbrowser.org/) there's an integrated translation system available from the language chooser (in the Options-menu). It even makes real "commits" to the code base, therefore they are available with your name in the code history (e.g. Update translation (cs)). But they won't appear right away (yet), I like to have an eye on them before uploading.
Status (as of version-2.0.8)