Hi @HeroadZ
Bart is trained in English so I don’t think fine-tuning it directly will help.  If you want to train a model from scratch in a new language then yes, you should train a new tokenizer. To train a new tokenizer checkout the tokenizers 15 library.
And both MBart and MT5 support Japanese so that would be a good starting point.
Another option is to leverage language-specific encoder only bert model (in your case  bert-japanese) to create a seq2seq model using the EncoderDecoder framework. See this notebook to know more about EncoderDecoder models
Leverage BERT for Encoder-Decoder Summarization on CNN/Dailymail 59