src/transformers/models/mbart/tokenization_mbart_fast.py/MBartTokenizerFast/__init__
class MBartTokenizerFast:    def __init__(
        self,
        vocab_file=None,
        tokenizer_file=None,
        src_lang=None,
        tgt_lang=None,
        additional_special_tokens=None,
        **kwargs
    ):
        super().__init__(
            vocab_file=vocab_file,
            tokenizer_file=tokenizer_file,
            src_lang=src_lang,
            tgt_lang=tgt_lang,
            additional_special_tokens=additional_special_tokens,
            **kwargs,
        )

        _additional_special_tokens = FAIRSEQ_LANGUAGE_CODES.copy()

        if additional_special_tokens is not None:
            # Only add those special tokens if they are not already there.
            _additional_special_tokens.extend(
                [t for t in additional_special_tokens if t not in _additional_special_tokens]
            )

        self.add_special_tokens({"additional_special_tokens": _additional_special_tokens})
        self.lang_code_to_id = {
            lang_code: self.convert_tokens_to_ids(lang_code) for lang_code in FAIRSEQ_LANGUAGE_CODES
        }

        self._src_lang = src_lang if src_lang is not None else "en_XX"
        self.cur_lang_code = self.convert_tokens_to_ids(self._src_lang)
        self.tgt_lang = tgt_lang
        self.set_src_lang_special_tokens(self._src_lang)