The language used in Japanese lawsuits is Japanese only.
In lawsuits, even if a presiding judge understands and speaks English, s/he will not allow you to use English. If you do not understand or speak Japanese, you have to be accompanied by an interpreter or a lawyer.
You also have to translate all the evidence (such as emails, messages and agreements) translated in Japanese if you want to submit it with the court.
That is also true in Japanese mediation proceedings.
However, since mediation proceedings are flexible, a court, especially the Tokyo Family Court, may prepare a mediator or judge who can understand English if any. And you may be able to talk in English in the process. But this not guaranteed. And at the end of the proceedings when a presiding judge reads out final agreed clauses to conclude the proceedings, it will be in Japanese. So you have to understand Japanese or bring your own interpreter if you intend to proceed with the process only by yourself.
If you wish to hire an English-speaking Japanese lawyer, contact us.