code: protected PropertiesHolder getMergedPluginProperties(final Locale locale) {
          return CacheEntry.getValue(cachedMergedPluginProperties, locale, cacheMillis, new Callable<PropertiesHolder>() {
              @Override
              public PropertiesHolder call() throws Exception {
                  Properties mergedProps = new Properties();
                  PropertiesHolder mergedHolder = new PropertiesHolder(mergedProps);
                  mergeBinaryPluginProperties(locale, mergedProps);
                  return mergedHolder;
              }
          });
      }
docs: Get a PropertiesHolder that contains the actually visible properties
  for a Locale, after merging all specified resource bundles.
  Either fetches the holder from the cache or freshly loads it.
  <p>Only used when caching resource bundle contents forever, i.e.
  with cacheSeconds < 0. Therefore, merged properties are always
  cached forever.