Format_tz allow to format a date, but if the format was '%B', it was always January regardless lang in context
Now, it possible to use babel, to get the date in the customer language.
So:
format_tz(object.date_order, format='short', context={'use_babel':1, 'lang': 'fr_BE'})
will return '5/01/16 22:20'
format_tz(object.date_order, format='short', context={'use_babel':1, 'lang': 'en_US'})
will return '1/5/16, 10:20 PM'
format_tz(object.date_order, context={'use_babel':1, 'lang': 'en_US})
will return 'Jan 5, 2016, 10:20:31 PM'
format_tz(object.date_order, context={'use_babel':1, 'lang': 'fr_BE'})
will return '5 janv. 2016 22:20:31'