I don't think there is a good solution for you. Except ISO format dates in plain text fields, but I understand the cultural problems you have with that obvious solution...
One other thing that you might want to examine is the possibility to enter complete dates. dd mm yyyy, but only display years.
There is a calibre template function that allows you to reformat dates, format_date. Perhaps that could be used somehow?
http://manual.calibre-ebook.com/template_lang.html