Basically fork the project, start translating and then send pull requests. We will then take the updated translation files and put them back into the main RHQ repo.
If you want to add a new language, copy the default file (e.g. Messages.properties) to Messages_xx.properties and then work on that new file (xx is the language code of the target language). Best is to probably comment out all lines with a hash mark first and then when you translate a line, remove the hash to comement it back in.
Also small contributions matter. You can make a change!
NOTE: Please read the following wiki article for some of the more technical aspects like file format, encoding and hints about IDE setup etc.