Steps to reproduce:
Open email, click on button "Markieren" ("Mark").

Actual results:
The button to flag an email is called "Markierung hinzufügen" (literally "Add marking") in German (and "Markierung entfernen" ("Delete marking")).
This is confusing because the "main button" is already called "Markieren" ("Mark") and is thus very similar to "Markierung hinzufügen" ("Add Markierung"). One therefore wonders what kind of marking is added when clicking on "Markierung hinzufügen".

Expected results:
Under the button "Markierung hinzufügen", analogous to "Als gelesen"/"Als ungelesen" (As read"/"As unread"), one would expect an option like "Als wichtig"/"Als unwichtig" ("As important"/"As unimportant"), since in German there is no common translation for "as flagged"/"as unflagged".
The expression "as important"/"as unimportant" expresses best what this function ist meant for.