Can you please check the translation of "short-cut-strings"?
I think it is not correct to translate "Ctrl" to "Strg"
With GUI Language set to German goto Bearbeiten - Einstellungen - Tasterturkürzel and press button "Alle zurücksetzen"
All shortcuts former prefixed with Ctrl will loose this. E.g. it will become impossible to type square brackets.
Reset Language to English. Restart Sigil, reset keyboard shortcuts once again, you will see the difference.