Hello @david_fiaty ,
May be your sting is coming from a js file so for translating the js file you need to remove the static content from pub/static folder and var/view_preprocessed folder as the js translation is saved in js-translation.json file. Please follow below steps:
rm -rf var/view_preprocessed pub/static/frontend/*
Then deploy the static content for that language and flush the cache.
Please also check the changes in private window.
If this helps you, please accept it as solution and give kudos.
Regards.