You can set the default locale of your survey in Survey123 Connect on the settings tab. Unfortunately, XLSForms uses the IANA language subtag registry, which only has a generic "English" language entry. There are a few variants in that list, but Survey123 only looks for actual languages, not variants, so there doesn't seem to be any support for English locales that have other date/time formats.
Personally, I have my browser and machine locale settings to always show DD/MM/YYYY, and most places it works. In Survey123 web forms, though, there seems to be a setting in the form that overrides my settings.
A really hacky workaround would be to set your form's default locale to something other than English. But then your month names and weekdays will all be in that language, and if you use multilingual forms, it's kind of a non-starter.
Date fields are really just integers under the hood, so it seems like it ought to be pretty trivial to set a default format for the fields to display. I would submit this as an Idea!
- Josh Carlson
Kendall County GIS