I have heard stories from others about their foreign credit cards both working and not working, so you probably just have to try it. As for mixing content, I found it is best to keep your Japanese Amazon and your local Amazon accounts separate. Do this by either using a different email address for each, or if you want to use the same email address, make sure you assign a different password to each account. For some of the Kindle tools (I can't remember if this is for the app or the Kindle device), if you use the same email address and password on both accounts, the Kindle app/device will decide for you which account it wants to connect to. You can avoid this default behavior by keeping either the email addresses or passwords distinct.