Reposting this from the Tales of the Rays discord:
Purchasing on Google Play:
If you're getting the "item not available in your country" on Google Play, you will need to set up an account for purchases on JP store.
IT IS RECOMMENDED TO DO THIS ON AN ALT Google account and NOT MAIN ACCOUNT because you will ONLY be able to pay for JP items with this account afterwards.
1. Go to https://pay.google.com/gp/w/u/0/home/settings . Make sure it is logged into your alt account (check the top right user icon)
2. Click the pen next to the "Country/Region" section and it will let you create a new payment profile in Japan. Use any Japanese address.
3. You can now select between the two payment profiles. Choose the non-JP payment profile and scroll to the bottom, click "Close payments profile" to delete it.
4. Clear the cache and data for the Play store and open again, you should now be in Japan.
5. Add your card to Google Play. It may show the Japanese address you put in for billing, but payments should still go through just fine. Note that there may be fraud checks with your card for beginning purchases, work with your card provider regarding that if so.
If the game is not switching to your JP alt during payment, remove other Google accounts from the device (you can re-add them later and contacts, etc will be intact)