You'll have to buy another Switch to have a second island - confirmed

[-Apples-]

I’m okay playing with one island. I once planned on having four islands, but managing them all will be a chore.

 

[MajinKitty230]

That stinks! But.. anything for Nintendo to make quick bucks huh?​

 

[RedTropicalPeachyFish]

that the different regions of animal crossing new horizons will be recognized as different games on the switch

Yeah that's actually true. The Switch will play all games from all regions just fine but if you have the same game twice but from different regions it will see the game as seperate. And it also makes seperate save files for them on your system.

I forgot about this!

Just a little update. NH in any regions are not the different version of the game but they're same game.
For example, you buy 2 games, one bought in UK and one in Japan. Say, you play the UK one with language setting "English" and that's your first NH play on your Switch. You switch the cartridge you bought in Japan. Then the island it loads is not the new island, it's the island you made by UK version. If you want to play the game in Japanese then you change the language setting to "Japanese" and it'll show the title "あつまれどうぶつの森" instead of "Animal Crossing New Horizons" and that's the only difference.