Described here: https://learn.getgrav.org/cookbook/twig-recipes#displaying-a-translated-month
put this file into user/languages of your site (create if it doesn't exist).
enables names of dates translated to german by appending |t.
example:
day of the week:
short {{ page.date|date('D')|t }},
long {{ page.date|date('l')|t }}
month:
short {{ page.date|date('M')|t }},
long {{ page.date|date('F')|t }}- assumes PHP returns english letters.
- for german not all keys are needed, because some are equal and
|tjust returns the input - feel free to add more languages