A bit late replying, but I had the same issue and wanted to share the solution. I tried the method above but it didn’t seem to make a difference.
Look for wp-content/plugins/custom-contact-forms/js/jquery.ui.datepicker.js
On or around line 55, look for
dateFormat: 'mm/dd/yy'
and change to:
dateFormat: 'dd/mm/yy'
This fixed it for me.
Cheers,
Simon