most electronics come with several choices, the difficult is as you say it's in japanese. if there is a menu window then slowly go through seeking the word english you might be lucky and it shows you can but try , click and see result. if gorgeous female steps out may i have first refusal. lol. good luck
ps you could learn japanese lol
My suggestion. Find a picture of the harness on eBay where it's being sold. Zoom in and you'll figure it out pretty easy from there. I'd do it but I'd need the harness in my hand.
Is it a detachable face unit? If so on the back of the stereo clean the contacts behind the face and on the unit on the dashboard besure your car is off when doing so.
you may have to find the serial number on the back of the radio if it's possible get the unit out and look on Google for car radio codes you should be able to get one free with the numbers of the unit
It would be best to go online with that model number looking for an owners manual. There would be the diagram showing what each color wire is used for.