This is cute but almost unplayable with a german keyboard, where Z+X are at far apart, X is left and Z right. Why not use WASD like most games? :(
It's not a perfect solution, but -- there are programs out there that let you remap your controls manually, I think. More accessibly, if you temporarily swap your keyboard's language settings over to US English, Z and X will be the two keys just to the right of left shift. I have my keyboard set up to be able to swap between entering English and Spanish, and in Spanish mode some of my punctuation turns into a quick way to add accents onto letters.