Note to self for typing the umlauts in my #Debian system:
Switch the input method to DE and then type:
' for ä
; for ö
[ for ü
For capitalization, just compose them with the Shift button as follows:
Shift+ [ for Ü
Shift+ ; for Ö
Shift + ' for Ä
Thanks to
@quincyQuincy ⁂ for guiding me.