Once you installed the server you will see a translation.js in the www directory.
Linux: /var/urbackup/www
Windows: C:\Program files\UrBackupServer\www
You could just change the strings in translation.de{} from German to your language (look up in the English version up top when necessary). You should then see all changes once you refresh the web interface (F5) and select German. Once finished you can send me that translation.js (martin@urbackup.org).
If you are starting with the english version, don't forget to translate the strings that have no english translation (because the strings themselves contain the translation). Those are all starting with "t". You'll find them e.g. at the German or Russian version.