Indeed, the account's billing address (as verified by card) is likely to the be universal indicator.
While in Italy last Christmas, a relative bought us a lot of Google Play credit.
Because it was bought in Italy, we could only claim it on a Google Play account registered to an Italian address. And now we can't change it (without losing the credit), choose or download any movie for it without being in Italy, or use it to buy from anything but the Italian Google Play store.
We told them that we very much appreciated the gift, and it SHOULD have been a wonderful idea, but not to bother next year.