The $400 annual payment is worth 260 Euros. This is an incentive to buy the "discounted" bond? LOL!
It gets really complex,
A 4% coupon is a 4% coupon. Not that complex.
It's REPORTED as income of 260 Euros. If it never gets converted from dollars to Euros, it's still $400 to the bond holder (at least for now).
Remember -- a European investor today is not making decisions based on the assumption that the U.S. dollar is (or ever will be) completely worthless . . . he's betting that the U.S. dollar is going to go up, down, up, down, etc. over different intervals in the future. And he's making investment decisions accordingly.