Dragon Quest Rivals Android/iOS/Switch - Setup Japanese eShop Switch Account
Step 1: Account Setup
To start you will need a different email address than your main eShop account, so head over to your favoirite email providter and setup a new email account. Then you will need to create a Japanease Nintendo account. Go to https://accounts.nintendo.com and create a new account from scratch using your new email address and set the location to Japan.
Step 2: Link the Account
Create a new account on your switch with any name you choose (Jpn Acct, Japan, etc). While creating the profile you wilbe asked to link the profile to a Nintendo Account, which you do using the info from the account that was created above. Or you can link the account via the profile settings under System Settings > Users.
After linking the new profile to the Nintendo account, your Switch will ask you to enter a confirmation code. This code will be sent to the email address that you used for the Japanese account above. Enter that code to complete the link.
Step 3: Japanese eStore
Once your account and profile are setup you can now browse the Japanese eStore, but beware that the store is mostly in Japanease, so you may have to browse the apps looking for the icon for Dragon Quest Rivals.
You only need to use the Japanese account to access the eShop - not to play the Japanese games that you download.