code: @Override
      public Collection<ResourceBundle> bundles() {
          Set<ResourceBundle> bundles = new LinkedHashSet<>();
          for (I18nExtension extension : extensions) {
              bundles.add(extension.bundle());
          }
          return bundles;
      }
docs: Retrieves the set of resource bundles handled by the system.
  
  @return the set of resource bundle, empty if none.