I’m going to have to mark this one as “Will not fix”, for now at least. I have added it to the list of things to investigate for the next version of kiwitrees (kiwitrees-nova).
The present code only validates dates entered manually by either tabbing to or clicking on another field, or using the “show” button. Pressing ‘enter’ tells the browser to submit the form immediately, which means before each date can be validated. It therefore reverts manually entered dates to the default format rather than ‘your’ language.