One more data point. Very similar to the experience of der_saeufer above. On my most recent trip to India, I noticed again that my US-region Google Wallet app switched to only showing me my loyalty cards and boarding passes in it, and no payment methods. However, the small icon for the wallet in the top pull-down options (Android) still showed the icon and last 4 digits of my default contactless payment card. Experimentally I tried using the phone for contactless payment a couple of times and it worked fine. It's just that the payment card is nowhere to be seen in the app while I'm in India. It also means that I couldn't change it to use any of the other cards stored in the app either.

App stayed in that "locked" condition during a short stop-over in London (no telling if it would have changed had I stayed longer). But a few minutes after landing in the US it reverted to showing me all my payment methods in the app just like before.

So I guess the only thing to be learned from this is, you *can* continue to use a US-region Google Wallet app for contactless payments in India, but be sure to set your preferred payment card as the default, as you can't change it once you're in India.