After installation of eZ Platform:

composer create-project --keep-vcs ezsystems/ezplatform application
bin/console doctrine:database:create
composer ezplatform-install

The back office language is installed accordingly to https://doc.ezplatform.com/en/latest/guide/back_office_translations/

composer require ezplatform-i18n/ezplatform-i18n-fr_fr

However, the new language is not selectable in User settings like described under https://doc.ezplatform.com/en/latest/guide/back_office_translations/#selecting-back-office-language

There is only the entry "English", even after clearing cache.