code: public void synchronize(NlsBundleDescriptor bundle) throws IOException {
  
      PrintWriter out = getStandardOutput();
      if (bundle.getMessages().isEmpty()) {
        out.println(bundle.getQualifiedName() + " is empty - noting to do!");
        return;
      }
      List<String> locales = getLocales(bundle);
      if (locales.isEmpty()) {
        getStandardError().println("No localized bundles for " + bundle.getQualifiedName() + " on your classpath!");
        return;
      }
      SimpleDateFormat sdf = new SimpleDateFormat(this.datePattern);
      String date = sdf.format(new Date());
      for (String locale : locales) {
        File targetFile = getTargetFileMkdirs(bundle, locale);
        synchronize(bundle, locale, targetFile, date);
      }
    }
docs: This method synchronizes (creates or updates) the localized bundles (properties). If a bundle already exists, it
  will NOT just be overwritten but the missing keys are appended to the end of the file. If no keys are missing, the
  existing file remains untouched.
  
  @param bundle is the bundle instance as java object.
  @throws IOException if the operation failed with an input/output error.