$ composer require mageplaza/magento-2-german-language-pack:dev-master
$ php bin/magento cache:flush

Package mageplaza_de_de is not a Magento language and will be skipped.
Nothing is removed.

MagentoFrameworkComponentComponentRegistrar::register(
    MagentoFrameworkComponentComponentRegistrar::LANGUAGE,
    'mageplaza_de_de',
    __DIR__
);