https://www.polygon.com/2018/9/19/17878620/nintendo-switch-online-famicom-games-nes-japanese-eshop-how-to

The Nintendo Switch isn’t region-locked, but you need to have a separate account to access the Japanese version of the eShop. Setting one up is a breeze — this is a good excuse to do so if you haven’t already — and the best part is that, if you have an account that’s “located” in Japan, you can download the Japanese versions of the NES games that come with your Nintendo Switch Online subscription.

It’s an easy process: Log into the eShop with your Japanese account, download the Japanese version of the Nintendo Switch Online app, and then jump back over to your primary account with the Nintendo Switch Online subscription to access it.