With the Kobo it depends on the account, not the device. If you have a Japanese Kobo account (country set to Japan, Japanese address in the billing information) you should be able to access Japanese books even if you're not actually living in the country. You can try that even now on the Kobo website and see if it works, before you decide if you want to buy the device
Same thing for Amazon - it's all based on the account. I'm still using the co.jp account with my old Japanese address on my PW3 and I have no problems accessing the store from abroad.