Corrected default values for MODELTRANSLATION_LANGUAGES in settings

The current default for MODELTRANSLATION_LANGUAGES in geonode/settings.py does not match that specified for the djang-modeltranslation app - see Section 2.3.2 of https://media.readthedocs.org/pdf/django-modeltranslation/latest/django-modeltranslation.pdf  or https://django-modeltranslation.readthedocs.org/en/latest/installation.html#modeltranslation-languages