Change language of form validation error messages

Recently Unbounce changed the forms to comply better with html5 standard including html5 error messages when validating e.g. required fields. This is very good.

But if I set the page language (or the form language) to e.g. “da”, why doesn’t this change the validation messages to danish as expected? Does Unbounce have another way to change this?

To change page language I use this script for header (works fine):
<html xmlns="" lang="da" xml:lang="da">

To change form language I use this script before body-end-tag (works fine):
<script> $("form").attr("lang", "da"); </script>

Best regards