Okay, one step closer. The external keyboard works now
(with the limitation that there is no key repeat, holding a key produces the alternative character like with the internal kb, and the modifier keys like shift and alt work like with the internal keyboard, too, not like on a regular pc where you hold down the key while typing).
However, when the correct layout for the external keyboard is active, the layout for the internal one is wrong (maybe US layout?). The only way I've found to fix that is to run

This, however, ruins the layout for the internal keyboard, too - help?