How to change the switch window key?
The language specific shortcuts are set up when the computer is first set up. In my case, the computer was set up in German and I only later switched it to English. Then the default hotkeys do not change. The desired behaviour (switching windows with CMD+' could be enabled by going to System Prefs > Keyboard > Shortcuts > Keyboard and then pressing "Restore defaults". In particular "Move focus to next window" changes then.