This tip answers following questions:
1. How do you I start an RCP application in locale of my choice
2. I am starting my application in say ja_JP but still all default widgets (ok, cancel button etc), preference pages, help, update screens etc are coming in English.
Answer to both these questions is pretty simple once you know 
In order to start an application in your desired locale you can either give a command line option (-nl=locale) or you can specify it in config.ini file (osgi.nl).
For more runtime options search for “Runtime Options” in plugin development guide.
Now answer to second question is “install language pack” and make sure they are included under dependencies of your feature/plugin. for ex to get prefence page or update screens in your language you would need org.eclipse.ui.workbench.nl1 and org.eclipse.update.ui.nl1 fragments.