def METHOD_NAME(cls, task, lang):
    prefix_size = int(task.data_cfg.prepend_tgt_lang_tag)
    prefix_tokens = None
    if prefix_size > 0:
        assert lang is not None
        lang_tag = SpeechToTextDataset.get_lang_tag_idx(lang, task.tgt_dict)
        prefix_tokens = torch.Tensor([lang_tag]).long().unsqueeze(0)
    return prefix_tokens