Most head units for the Japanese market can’t switch to other languages.
If it’s a JDM Alpine deck, your best option is to swap it for a European or Australian one. The wiring should match. Keep in mind that Japanese radios use different frequency ranges than US or European ones.
No judgment, but why did you import a Prius from Japan?
Do you have the owner’s manual? It might help.
Does the radio even work?
Easy fix—just learn Japanese!