Create a single source for language data

[etj]

At the moment we have scattered language info:

• a list of languages with 2-letter codes and mixed localized labels: https://github.com/GeoNode/geonode/blob/5.1.0/geonode/settings.py#L191-L224
• a list of languages with 3-letter codes and English labels: https://github.com/GeoNode/geonode/blob/5.1.0/geonode/base/enumerations.py#L318-L434
• extra language info: https://github.com/GeoNode/geonode/blob/5.1.0/geonode/settings.py#L228-L253

We need to centralize these information in order to provide:

• code translation from ISO639-1 (two-letter codes) and ISO639-2 (3thee-letter codes), and vice-versa.
• labels for each language expressed in the language itself
• localized labels

This file could be a good starting point:
https://github.com/haliaeetus/iso-639/blob/master/data/iso_639-1.csv