requires_model_validation deprecated in Django 1.7

Replaced with requires_system_checks, for details check: https://docs.djangoproject.com/en/1.7/releases/1.7/#django-core-management-basecommand
Get list of languages from settings to pass to _get_po_paths to prevent iteration over None object
Merge remote-tracking branch 'origin/patch-1'
Fixes compatibility with django 1.7