Append an item to the list. If there is not enough sentences in the
        list, then the list is extended as needed.

        There is no control made to make sure that the key is consistent.

def append(self, item: TransItem):
        """
        Append an item to the list. If there is not enough sentences in the
        list, then the list is extended as needed.

        There is no control made to make sure that the key is consistent.
        """

        if not (1 <= item.index <= settings.I18N_MAX_SENTENCES_PER_GROUP):
            return

        if len(self.sentences) < item.index:
            for _ in range(len(self.sentences), item.index):
                self.sentences.append(Sentence())

        self.sentences[item.index - 1].append(item)