Ok, David, you got me poking around some more. Easier than you think (uh, motivating me to poke around and setting up the JP paypal button).
Login to your Paypal account via the Japanese site (
https://www.paypal.com/j1, then create the button. Simple.
Trick for me was to use IE that isn't set to English as default language to create the button. Plugged it into a simple html page for testing, and Voila! Japanese payment screen.
You can see the test button here:
http://www.tcobag.com/paypal-jp-test-text.html
Hope that helps!
let me know if it works okay for you.
Be well,
Allen