The resource bundle lookup searches for classes with various suffixes on the basis of (1) the desired locale and (2) the current default locale as returned by Locale.getDefault(), and (3) the root resource bundle (baseclass), from lower-level (more specific) to parent-level (less specific).
So the default in your case depends on:
1. Which language you put in your root resource bundle (most people use English in the root bundle, since it is most likely to be understood by somebody with a native language not covered by a resource bundle)
2. What your default locale is.
So, you need to ensure that you have the root resource bundle (no locale extension) in addition to your language-specific ones, and you need to take the default locale into consideration.
If you are writing an application that will be run directly on the client (not from a server), the problem is easy to solve: the default bundle will be used in all cases where the user's default locale does not match an existing resource bundle.