Hi Craig,
Thank you for your prompt reply.
I’ve done it 
The translation is applied to the mini cart too.
I found this article: https://magento.stackexchange.com/questions/193830/magento-2-how-to-translate-the-minicart?rq=1
I removed the file /pub/static/frontend/Magento/luma_child/en_GB/js-translation.json cleared caches like Evgeny Levinsky adviced and it worked.
We learn things every day haaa !
I hope that it will be be useful for someone.
Regards,
Pawel