In Java how do I find out what languages I have available my Resource Bundle

I have some resource bundles packaged in my main jar

widget_en.properties
widget_de.properties

I retrieve a resource bundle based on my default locale as folows

ResourceBundle.getBundle("widget", Locale.getDefault());

But I want to present the user with a list of available languages supported so that can select a language that may be different to their computers default

But I can't find a method in ResourceBundle that would list available locales, I don't want to hardcode a list as I may forget to update it when another resource bundle is added.

EDIT

As I only resource bundles for different language (I dont have country refinements) so I have got generated a list by iterating through all the known language codes and check for each one as a resource.

String[]langs = Locale.getISOLanguages();
for(String lang:langs)
{
      URL rb = ClassLoader.getSystemResource("widget_"+lang+".properties");
      if(rb!=null)
      {
            System.out.println("Found:"+rb.toString());
      }
}