In the manual I can find the key mapping for german and english keyboard layouts. Is there also a function to detect which keyboard layout the player is using? In the german layout for example the open bracket is over the eight but in the english layout it is over the nine. How can I adress the open bracket for both versions correctly?

Theres something called virtual keycodes to abstract this problem and pass it over to the OS instead. You dont have to tinker arround with the layouts when coded right.
Anyways, there are windows APIs to do what you want: GetKeyboardLayout

How does GetKeyboardLayout work and how can I use it in lite-c?

https://msdn.microsoft.com/de-de/library/windows/desktop/ms646296(v=vs.85).aspx

You just call it with parameter 0 and check the set bits of the return value.

Thanks for the info!