Create a language pack
Create directory app/i18n/Firstfound/en_GB
Issue command:
bin/magento i18n:collect-phrases --output "app/i18n/Firstfound/en_GB/en_GB.csv" --magento
Make changes to the file and issue:
bin/magento i18n:pack app/i18n/Firstfound/en_GB/en_GB.csv -d en_GB