I've had a look and can't find any option to change the language (which is why I assumed you were looking into changing the firmware).
Also, I know that the UK version of the Firmware allows you to change the orientation so can only assume that the US version also allows it.
PRSCustomizer may be your answer. I am sure that it backs up your existing firmware and allows you to install a different version. If you put UK/US firmware then I am sure you would get English rather than German!
Have a look through the thread and ask some questions there...
https://www.mobileread.com/forums/showthread.php?t=27177