Setting the country locale for anonymous web users in OO?

The enviroment is the latest Nextcloud with the latest community version of Onlyoffice and we are in Germany. So Nextcloud is generally set to German language.

I have a table containing dates. When I’m logged in the dates are shown in German format like “1. April 2020” by Onlyoffice.

Now I allow editing for anonymous web users by sharing a link. Onlyoffice has German menus for the web users. But all dates become americanized for them, in this case “April 1st 2020”.

How can I set this also to German as a default?