Same situation last week at the Westin Vendome Paris, except I was the guest redeeming the card! Luckily it was not too busy, but the front desk guy did have to check with the back office a couple times so it added at least 10–15 minutes to the process. It was the first time he’d seen one of these. But ultimately he converted the $50 to an equivalent Euro amount and applied it to my bill. So good news is they can work outside the US, but be ready to be very patient.