code: public boolean isRegisteredLanguage(String language) {
          String lang = getLanguageOrDefault(language);
          return getRegisteredLanguages().contains(lang);
      }
docs: Returns true if the exact language is a registered language in this
  application.
  <p>
  For example, this application may have a Portuguese translation
  (messages_pt.properties) but not a Brazilian Portuguese translation
  (messages_pt-BR.properties). In this case, pt-BR is not a registered
  language, though it may be a supported language by the 'pt' language
  component.
  </p>
  
  @param language
  @return true if the language is registered