Delta international baggage?

The 23 kg (50 lbs) is not a total weight, it is maximum weight per bag. Your first bag, weighing no more than 23 kg, is free. It doesn't matter whether it weighs 4 kg or 23 kg, it's still one bag. Your second bag, also weighing no more than 23 kg, will cost you 50 Euros online (55 at the airport). When you return, you get to pay again, but in US$, with exactly the same provisions.