Install Eclipse IDE : http://www.eclipse.org/downloads
Install ResourceBundle Editor (RBE) : http://docs.ofbiz.org/x/5QM#RBE
Install Subversion (svn) plugin in Eclipse aka Subclipse : http://subclipse.tigris.org
Link to the repository with Subclipse : http://docs.ofbiz.org/x/mgM
Make a project from this repository with Subclipse (Check Out)
At this point you should be able to do any translation with *ease* (RBE will create new *_YOUR_LANGUAGE.properties files automatically for you)
For RBE setting best practices please see http://docs.ofbiz.org/display/OFBIZ/Plugins#Plugins-RBE