The js-translation.json file is autogenerated. You should delete it and delete the cache, and the system will recheck your new interface strings and the dictionary used and will create a new js-translation.json file.
The js-translation.json file is generated correctly, it is also loaded on the frontend side and even stored in local storage. But it seems that at the mentioned location in catalog-add-to-cart.js the translations are not yet loaded, so $t always returns the original (English) string. This is clearly a bug, not something that we could fix in the language pack.