Which country/region should I select in AppStore

At this point, you don't appear to qualify for an account in any region.

To open an App Store account you must be a legal resident of the region in which you live. You must provide a payment method issued by a bank in that region with a verifiable billing address in that region.

An alternative is to buy an iTunes/App Store gift card for the region in which your are living and redeem that gift card as a credit balance on your account as your payment method.

M1 Mac mini, macOS 14.1.1;  Watch, series 7 GPS/cellular, watchOS 10.1.1; iPad Pro 2nd Gen, iPadOS 17.1.1; iPhone 12 Pro, iOS 17.1.1; TV 4th Gen, tvOS 17; HomePods, audioOS 17.1.1 and Airport Express w/ Airplay 2

A fellow upper-level Apple Support Communities volunteer called Apple Support this morning. Apple Support confirmed that what I have stated is correct.

You must be a legal resident of the country/region and if you intend to use a credit/debit card as your payment method, it must be a card issued by a bank in that country/region with a verifiable billing address in that country/region.

Apple Support confirmed that if you intend to use a credit/debit card as your payment method, it must be a card issued by a bank in that country/region with a verifiable billing address in that country/region.

Change your Apple ID country or region - Apple Support

What to do before you change your country or region

#4 Make sure that you have a payment method for your new country or region.

Apple does not say anywhere that you must have a payment method issued by a bank in that region or anything about billing addresses. However, you may find if you set your account to a country that the address layout has to match the entry options for a country.

You can buy an iTunes card, but for many thing Apple will require a valid form of payment such as a credit or debit card even if an app is free. Account balance alone may not suffice.

As for country, you can read the terms: Apple Media Services Terms and Conditions - Legal - Apple Media Services - Apple

- "Our Services are available for your use in your country of residence (“Home Country”).

Since you now reside in country B, you should have that set as your country. Unfortunately you will have to deal with the language issue in your own way. :-) You will also have to see what you can do about a payment method. I know I use my account with no payment method (but I also do not download anything that costs money). Many people on this forum say you absolutely MUST have a card but I have never found this to apply in my case - maybe I just have an old account that works under different rules.

Again, nowhere does Apple actually say you must have a payment method from that country. Apple's online documentation is by no means thorough, but since this question has been around for 20 years now you would think that if you had to use a card from the same country then they would simply say that in a short, clear sentence. Not saying that to me implies that it isn't such a simple matter. They may impose this indirectly by other means but in fact what Apple does say in its official documentation implies you can use payment methods from other countries:

The statement below was removed from the November 10, 2021 version of the payment method document Payment methods that you can use with your Apple ID - Apple Support

"Make sure that the billing address for your payment method matches the billing address for your Apple ID. In some cases, you might need to call your bank or financial institution to authorize international or online purchases. You might also need to confirm that the mobile number associated with your payment method is correct." However, Apple doesn't even say that anymore. All they now say is from If your payment method is declined in the App Store or iTunes Store - Apple Support

"You might need to ask your financial institution to allow international purchases on your debit or credit card."

Sometimes something one person posted on this forum gets repeated so many times by others thinking it was the official word that it eventually gets believed by a bunch of people as being such, and regarded as the true and final word. And (not to be mean to Apple) I have gone into an Apple Store and asked a question, told that something wasn't possible, and when I went home and did more in-depth research (and tried it myself) it certainly was.

For Family Sharing you must have an account set to the same country as your other family members.

Have you changed your country settings to country B and Apple Pay from country A still works? Or is your country still set to A?

While you are supposed to be resident (and physically present) in your "Home Country" (to which you ID is set), Apple doesn't actually require you to prove this. However, it used to be thought that having a payment method billed to you in a new country pretty much enforced this requirement (although-as I have said-this was not outright stated by Apple).

That does not say that it has to be from that region, just that the one you choose must be valid in that region. If you were using PayPal in your old country and PayPal is not valid in your new country then it won't work.

Was this a debit card from country A linked to your account now set to country B?