code: public void activateLanguageModelRules(File indexDir) throws IOException {
      LanguageModel languageModel = language.getLanguageModel(indexDir);
      if (languageModel != null) {
        ResourceBundle messages = getMessageBundle(language);
        List<Rule> rules = language.getRelevantLanguageModelRules(messages, languageModel);
        userRules.addAll(rules);
        updateOptionalLanguageModelRules(languageModel);
      }
    }
docs: Activate rules that depend on a language model. The language model currently
  consists of Lucene indexes with ngram occurrence counts.
  @param indexDir directory with a '3grams' sub directory which contains a Lucene index with 3gram occurrence counts
  @since 2.7