Please note that we have a new system for translations in trunk, no plugin like RBE is required anymore (except in release4.0). Look in the config directory of the example component to see how it is done.
Install Eclipse IDE : http://www.eclipse.org/downloads
Install an XLM editor (Oxygen, Buddy, WTP, etc.) : http://docs.ofbiz.org/x/5QM#Plugins-XML
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.
The best approach to follow will be easy to see if you understand how the UI labels system works.
Bottom-up point of view
The basic concept of the multiple UI label property files is this: start off with the most generic, most often used labels, then build upon that with application-specific labels. The CommonUiLabels.xml file contains labels common to all components. The EcommerceUiLabels.xml file contains labels specific to the eCommerce component. Doing things this way reduces redundancy and it allows an application to redefine labels found in the common (or shared) label files.
If a specific label appears in more than one property file, it is because the label is being redefined. And this definition overrides "parent" definitions. You can see how the parent relation is défined for each component by looking at its "main-decorator" screen in CommonScreens.xml. Specific maps come first, then come more common maps added for shared labels. In other word the most common are at the bottom of the stack.
The bottom line is, it is better to add translations to property files than it is to delete labels. Deleting duplicate labels makes sense at first glance, but problems could come up if there was a specific reason the duplication was needed (for instance to avoid an inter-application dependency. For instance EcommerceYourShoppingCartEmpty may be present in OrderUiLabels.xml to eliminate the need for the Order Manager component to load/use the ECommerceUiLabels.xml file.)
Top-down point of view
An interesting explanation from Adrian : https://issues.apache.org/jira/browse/OFBIZ-1698
Exernal point of view 
This link may be of great interest too : Standardizing and Internationalizing OFBiz User Interfaces . Beware : the part about ".properties" files is deprecated in OFBiz since February 2008 (we use now XML UTF8 files), still valid in release4.0 though)