code: function parseLanguages(book) {
      var logger = book.getLogger();
  
      return parseStructureFile(book, 'langs')
      .spread(function(file, result) {
          if (!file) {
              return book;
          }
  
          var languages = Languages.createFromList(file, result);
  
          logger.debug.ln('languages index file found at', file.getPath());
          logger.info.ln('parsing multilingual book, with', languages.getList().size, 'languages');
  
          return book.set('languages', languages);
      });
  }
docs: Parse languages list from book
  
  @param {Book} book
  @return {Promise<Book>}