Implement fallback language for PyCSW to avoid warnings

[etj]

pycsw recognizes only a few languages:
https://github.com/geopython/pycsw/blob/7fc81b42bfdc5b81250c24887fd6a66032a6b06e/pycsw/core/config.py#L50-L54

        self.languages = {
            'en': 'english',
            'fr': 'french',
            'el': 'greek',
        }

GeoNode, as of master, forces pycsw to use its own language:

geonode/geonode/settings.py

Line 1106 in cff1436

"language": LANGUAGE_CODE,

If the GeoNode language is not allowed in pycsw, there will be lots of warning in the log, making it hard to spot real issues.

Here the line raising the error
https://github.com/geopython/pycsw/blob/2.6.1/pycsw/server.py#L182

It would be useful to replace in GeoNode this line

            "language": LANGUAGE_CODE,

with

            "language": LANGUAGE_CODE if LANGUAGE_CODE in ('en','fr','el') else 'en'