With a workaround:
{% set x = state_attr('automation.giessen_pumpe_1_neu_2', 'last_triggered')) %}
{% set day_g = ["Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag", "Sonntag"] %}
{% set month_g = ["Januar","Februar","März", "April", "Mai", "Juni", "Juli", "August", "September", "Oktober", "November", "Dezember"] %}
{{ day_g[x.weekday()-1]}} {{ x.day }} {{ month_g[x.month-1] }} {{x.year}}
Gives: Mittwoch 9 November 2023
I’m not German, so no idea if this is the correct order for you 
But you can set the order by editing the last line. The -1 for weekday and month is because lists start at 0.
Stolen from: Get weekday in my local language? - #6 by petro