code: @Override
      public String getString(String resourceKey) {
          try {
              return bundle.getString(resourceKey);
          } catch (MissingResourceException mre) {
              return resourceKey;
          }
      }
docs: Looks up and returns the internationalized (i15d) string for the given resource key from the
  ResourceBundle that has been provided during the builder construction.
  
  @param resourceKey the key to look for in the resource bundle
  @return the associated internationalized string, or the resource key itself in case of a
  missing resource
  @throws IllegalStateException if no ResourceBundle has been set