code: function parseMultilingualBook(book) {
      var languages = book.getLanguages();
      var langList = languages.getList();
  
      return Promise.reduce(langList, function(currentBook, lang) {
          var langID = lang.getID();
          var child = Book.createFromParent(currentBook, langID);
          var ignore = currentBook.getIgnore();
  
          return Promise(child)
          .then(parseConfig)
          .then(parseBookContent)
          .then(function(result) {
              // Ignore content of this book when generating parent book
              ignore = ignore.add(langID + '/**');
              currentBook = currentBook.set('ignore', ignore);
  
              return currentBook.addLanguageBook(langID, result);
          });
      }, book);
  }
docs: Parse a multilingual book
  
  @param {Book} book
  @return {Promise<Book>}