How do I add an international credit card to Apple Wallet when country selection is unavailable?

The region is changed for the iPhone, not directly in the Wallet App, but you should not need to change the region.

If the card is supported for Apple Pay by its issuer and your device is set to a region that supports Apple Pay even if its different to the card's region it should be added normally.

Some card issuers and banks however, require you to start the process from their own App.

When adding a card to the Wallet App, region details are not requested nor required.

When adding a card to Apple Wallet, your iPhone reports certain details to the bank and the Payment Network Operator (Visa, MasterCard, American Express etc.). These details may include iPhone’s location, region iPhone is set for, and region you Apple Account is set to. This information, along data, is used by the bank and PNO to evaluate certain risk factors.

“Information may also be provided by Apple to those entities to enable Apple Pay, determine card eligibility, set up your card with Apple Pay, and to prevent fraud, including:

• Your credit, debit, or prepaid card number
• The name and billing address associated with your Apple Account
• General information about your Apple Account activity (for example, whether you have a long history of transactions within iTunes)
• Information about your device and, if using Apple Watch, the paired iOS device (for example, a device identifier, phone number, and the name and model, for both your Apple Watch and paired iOS device)
• Location at the time you add your card (if you have Location Services enabled)
• Account or device history of adding payment cards
• Aggregated stats relating to the information from payment cards you’ve added or attempted to add to Apple Pay”

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Please follow this path to change your region for Apple iPhone and Wallet,

iPhone > Settings > General > Language and Region > Region