translate.google.com shows a German keyboard with y and z transposed, the umlauts and so forth. not only can one mouseclick the keys on the screen keyboard, the amazing thing is that my REAL KEYBOARD is transformed so I can easily type umlauts, much easier than the Duolingo system.
also on a MAC, you can just enable a german keyboard input, and then your keyboard behaves like a german one. (SystemPref->Keyboard->inputSources). Swtiching between English/German is then using cmd-space. However, replacing "z" with "y" broke me eventually and I started using the option-u for umlaut, option-s for eszett.