The locale of Eclipse can be set at start up via eclipse.ini
adding this line:
-Duser.language=<locale>
But how can I get the locale in an Eclipse RCP application (plugin)?
The same way as in every java application:
Locale.getDefault();
Have a look at "Using a Default Locale":
http://www.oracle.com/technetwork/articles/javase/locale-140624.html?ssSourceSiteId=ocomen#using
There is also the LocaleProvider service, depending on your needs:
http://help.eclipse.org/luna/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Freference%2Fapi%2Forg%2Feclipse%2Fosgi%2Fservice%2Flocalization%2FLocaleProvider.html