translation.E004) You have provided a value for the LANGUAGE_CODE setting that is not in the LANGUAGES setting

In Django 3.0 you should use LANGUAGE_CODE exactly as mentioned in LANGUAGES list located in django/conf/global_settings.py

The list is a list of two-tuples in the format (language code, language name) – for example, ('ja', 'Japanese'). This specifies which languages are available for language selection.

So, in your case, it will be LANGUAGE_CODE = 'en-gb'

For French-speaking people, you have to change 'fr-fr' to 'fr'