Localization in RCP plugins is usually done by including translations in a property file and using their id in plugin.xml.
I spend some time lately trying to understand why some translations were not used in my application althoughthey were correctly displayed in PDE.
Answer : when using plugin.properties, you MUST include the following line in the plugin manifest :
Bundle-Localization: plugin
Otherwise, PDE will show the translated text, but your app won’t.
Strangely, I had to edit the manifest manually because I couldn’t find a way to enable plugin localization in PDE. Some other plugins of my application already had this line, but I don’t remember adding it. Maybe Eclipse templates have changed since I created my first plugins (using Eclipse 3.1).