Restore missing LanguagesSetting.get_fallback_language() method
Restore missing LanguagesSetting.get_fallback_language() method

This broke django-fluent-pages already.
CC: #78

ellmetha/django-parler,defivelo/django-parler,jrief/django-parler,jrief/django-parler,skirsdeda/django-parler,edoburu/django-parler,imposeren/django-parler,skirsdeda/django-parler,django-parler/django-parler,zhangguiyu/django-parler,edoburu/django-parler,ellmetha/django-parler,defivelo/django-parler,zhangguiyu/django-parler,imposeren/django-parler,django-parler/django-parler