I think your problem is connected with cronjob locale settings:
you can do instead is create (if not already present) the file /etc/environment and add the following line:
LANG=pl_PL.UTF-8
change your command:
class Command(BaseCommand):
can_import_settings = True
def handle(self, *args, **options):
# Activate a fixed locale, e.g. Polish
translation.activate('pl')
# Or you can activate the LANGUAGE_CODE # chosen in the settings:
#
#from django.conf import settings
#translation.activate(settings.LANGUAGE_CODE)
# Your command logic here
# ...
translation.deactivate()