It came up in a FB post recently, so I thought I’d just highlight it: Seller central is in English too! Scroll to the bottom and there is a drop down where you can select English or Japanese. There is a related pro tip, but I’ll leave that for a later time!
*This post originated from a post in the Amazon Japan Facebook group, “Amazon Japan PL”. The information may well have changed by the time you read this. Please check the DATE of the post before you make any decisions based on this information!