Date responses in Survey123 display as mm/dd/yyyy when accessed from the browser even when Windows 10 regional settings are set to European formatting (dd MMMM yyyy).
To change the date/time format on Survey123 web form, user would need to go to profile setting to set the language. Survey123 website follows the same logic as arcgis online to modify the langauge setting.
Please read the online doc to see how to change the language with your log in account:
https://doc.arcgis.com/en/arcgis-online/reference/set-language-region.htm
Once the langauge is set in user profile, open survey123 form will display all the content in the langauge you set, including the date/time format in the survey form.
Another way to override the language or locale on the survey123 website/form is modify the URL to include the lang setting without changing the profile language.
For example, set the language to German on Survey123 website, just add ?lang=de to the URL link and the web page content will display German, included German date/time format.
http://survey123.arcgis.com/surveys?lang=de
You can set this to the survey web form as well to override the survey form locale to German, such as
https://survey123.arcgis.com/share/c26ebe61769f4c68bb998c4c72e24c57?lang=de