I use QUERTY and UK/US keyboard layout, and the just memorised the Option key combos for the åäö etc. was less hassle for me.

Under linux I do the same, but under windows I have to constantly switch the layout to german, type ä, ö, ü or ß and switch back to english.

That's not the problem though; the annoying thing is when a PC is configured to only have the German layout available, so I constantly mix up Z and Y

This situation got me. Maybe I was rather tired when I did this. It was when one of the letters in my bank's password is a Z but I keep hitting the Y key. After 3 attempts, I got locked out and had to wait until the Monday to contact the bank!

Or you could memorise the ascii codes and type e.g. alt+[keypad] 0248 => ø.

Edit: Is that what GP Bjelkeman meant by "option key combos"?

On macos you can press <Option-u> followed by a vowel and you will get the corresponding Umlaut. (üäö) Additionally there are other combinations for accents and stuff like that.

Thanks, that's good to know!