Hi, I started having the same problem on my computer recently, for no reason. I also have JP Japanese (Japan) installed (along with French, which is also the language of my OS) however I've had it installed for more than a year.
Did you eventually find a solution to your problem or does someone have ideas now given that we both have Japanese installed?
Thanks
PS: you can remove it by installing EN Enlgish (United States) and deleting it, but it will come back anyway next time you restart your computer or use the alt+shift shortcut.