docs: adds docs for LANGUAGE_CODE and BABEL_DEFAULT_LOCALE

[pomegranited]

Documents how LANGUAGE_CODE is used to initialize Superset's BABEL_DEFAULT_LOCALE, with instructions for how to customize BABEL_DEFAULT_LOCALE if needed.

Relates to: openedx/tutor-contrib-aspects#1017
Part of: #298

Preview the changed section here: https://docsopenedxorg--308.org.readthedocs.build/projects/openedx-aspects/en/308/technical_documentation/how-tos/production_configuration.html#superset-localization

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