code: @Override
      public String etag(Locale locale) {
          String etag = etags.get(locale);
          if (etag == null) {
              // We don't have a stored etag, that means we don't have messages. We returns 0.
              // There is a potential race condition here:
              // We retrieve the etag get 0, but when we retrieve the messages, we get messages. The browser receives 0
              // as etag, which will not match the next request. It's should not be too critical as it will just send
              // the same content a second time.
              return "0";
          } else {
              return etag;
          }
      }
docs: Retrieves the ETAG for the given locale.
  
  @param locale the locale
  @return the computed etag, must not be {@code null} or empty