How can I buy this game once it's out? So far, I understand it's JP only.
For Switch, you need to create an account that is for the Japan region. that gives you access to the Japanese eshop and then you can buy this game. i believe you'll need to buy a Japanese Eshop card though, i don't believe they take any credit/debit cards that aren't from that region. I'm sure there is some good youtube videos out there that give you a step by step tutorial on all this stuff. https://www.youtube.com/watch?v=4R4IQ8yOt8w maybe this one, didn't watch it.
If you want physical (I think they are doing physical on this game), you can buy Japaneses games from places like play-asia.com or Amazon.jp. No mention yet if they plan to include english in these versions, so I'd double check that.