Use localized datepickers for date input fields in Grappelli

Hi,

thank you for sharing & maintaining this great, hiqh-quality, useful Django package.

I found some support for localized (i18n) datepickers in Grappelli, but they did not seem to work for me. So, I created this small patch. I'd be more than happy if you merged it. 

I used {% if LANGUAGE_CODE %} to test for i18n support, but I don't know if this is the best way.