According to the Official Animal Crossing: Pocket Camp FAQ, you can change your country settings once you linked Pocket Camp to a Nintendo Account.
Pocket Camp is now available worldwide, region switch Pocket Camp to your region is highly recommended.
If you have not yet registered a Nintendo Account, you can do so here:
Start the Pocket Camp app and click Nintendo Account Link on the home screen:
Click Create/Link Nintendo Account, login to your Nintendo Account when prompted, and agree to link your account.
Change the country setting for your Nintendo Account by selecting "Edit Profile" in the Profile Setting:
Your Pocket Camp country setting will be updated automatically.