Sorry to say Apple makes it real hard buying non-US Apple gear here in the States as well as makes it hard for foreigners getting US gear from the EU or Asia locations.
With that said, you can still buy a US system and still have german keys on the keyboard.
Key cap covers or overlays are both good answers here, but, they do add bulk to the keys which will rub on your screen when you close the lid.
Two other alternatives is to pop off the key caps and replace them with the german key caps (as needed) here is a company that sells replacement key caps LaptopKey.com
The other choice is to use an external german bluetooth keyboard leaving the laptops keyboard as is. Getting the key board before you come here. Then using the system's keyboard settings to matchup with the key board with the language you want to use.