cd <your magento path>
composer require bns/magento2-language-en-gb:*
php bin/magento cache:clean
To update package
cd <your magento path>
composer update bns/magento2-language-en-gb
php bin/magento cache:clean
Download file: https://github.com/bns/magento2-language-en-gb/archive/master.zip
cd <your magento path>
Create folder your-magento-path/app/i18n/bns/en_gb and extract zip into path.
php bin/magento cache:clean