CommandWithTranslation::get_installed_languages() currently assumes that translations for en_US are always installed.

$ wp language plugin install my-plugin en_US
Language 'en_US' already installed.
Success: Installed 0 of 1 languages (1 skipped).

But that's not always true. For example if you have a plugin which is by default provided in a different language than English. While translate.w.org doesn't support en_US as a locale, a custom implementation can support and hook into core filters to tell WordPress that there are translations for en_US.

Worth nothing that I can update en_US translation via WP-CLI once installed via wp-admin/network/update-core.php.