This is more of a “note to self”: After enabling FileVault on my MBP during installation, the input field for entering your FileVault password is using the US keyboard layout. That, in combination with an actual German keyboard, is good for some fun during your next reboot – of course only if you’re using a strong password, which includes special characters.
To be able to change the keyboard layout at boot, you have to tick the box as seen above.