docs: String messageFromFile(Locale locale, String filename, String relatedProperty) {
      String result = null;
      String bundleBase = propertyToBundles.get(relatedProperty);
      if (bundleBase == null) {
        // this property has no translation
        return null;
      }
  
      String filePath = bundleBase.replace('.', '/');
      if (!"en".equals(locale.getLanguage())) {
        filePath += "_" + locale.getLanguage();
      }
      filePath += "/" + filename;
      InputStream input = classloader.getResourceAsStream(filePath);
      if (input != null) {
        result = readInputStream(filePath, input);
      }
      return result;
    }