The date_date_string() should print locale dependent. So if you are outside US it should be DD/MM/YYYY.
But you can use date_datetime_stringf() to format it like you want. The formats can actually be found here:
http://www.cplusplus.com/reference/ctime/strftime/, as the function is not currently documented in ENIGMA wiki.
So if you want DD/MM/YYYY it would be like this:
date_datetime_stringf(data, "%d/%m/%y");