There are two German language settings in Nextcloud. “Deutsch (Persönlich: Du)” or “Deutsch (Förmlich: Sie)”. Just change the language of your Nextcloud to the second one. Then the mails will also be sent in the formal “Sie-Form”.
You should be able to change the language by clicking on your avatar at the top right in the web-interface and then click on “Einstellungen”. The language settings are in "Persönliche Einstellungen. If you don’t have any language settings there or can’t change them, an administrator probably has enforced the language settings for all users…