Sorry, Germany. YOU LOST THE EFFIN’ WAR. When you a lose a war that completely, the winners get to loot you. All that stuff belongs to whoever has it now.
And why do nations agree to international treaties and conventions, then? It´s illegal to take art from the losing nation. So, it´s very well Germany´s right to get the art back. Germany lost 1/4 of its territory as a “compensation”.