It is usually labeled Ctrl on keyboards which use English.
German layout uses Strg as required by the German standard
I would have thought the game supported other languages, I mean at least the Ctrl key for Germans would seem common, and nobody has ever mentioned this before. If you want, you could try downloading Autohotkey.com and see if you can set it to trigger Ctrl when you press Strg. AHK is a simple keyboard tool that allows you to change keyboard commands for gaming macro.