plasma-mobile is for mobile - I don’t think there is any settings specifically for external keyboard.
Though I have a pinephone pro with keyboard case - it is more a gimmick - IMO.
You can use localectl to setup keyboard layouuts.
man localectl
Read the wiki page on the keyboard - PinePhone (Pro) Keyboard - PINE64 - the is a few don’t ever do e.g. like attaching usb device to your phones usb-c port - which is a shortcut to brick your phone.