Zend_Date takes locale as a parameter. So you can also try:

$myDate = new Zend_Date(strtotime('2007-07-13'), false, 'de');

and this should convert dates to their localized formats. Sorry but I am not too sure about using the localized constants like DATE_LONG to automatically make dates in their locale format. Perhaps use the above and play around with toString() to see what you can get.

I am also now working on a multi-lingual site and am playing around with dates, so if I come across anything I will post it.

Cheers