How hard is it to implement such a feature?
very hard, since it’s not a coding problem but a data problem. Translators need a lot of data to be useful that mostly isn’t available for free. Mozilla is working on it for their browser but i’m not sure how good it is.
Did you know that the search engines on Firefox are sponsored too? Even if you don’t use those specific search engines, it supports firefox development. Without them firefox would be dead. It’s not like they’re selling your personal data.
Anyway there must be a way to disable it. did you search?