Does anyone know if this is used by any program to check if you are actually looking at the screen? I could imagine it being used for some blackmirror type stuff

They should at least show a prompt asking if the user wants to use the first language in their accept-language or the one from their IP.

Showing modal prompts on an initial page visit is an easy way to annoy users, though. A language selector in some prominent location will do just fine.

You could get a free email from a provider you trust. I have my domain's email in protonmail and use an email from another provider for my domain registration. I only use that email for important accounts, specially ones that I need to be able to access to control my domain (domain registration, bank, DNS/VPS hosting, etc)

Looks very cool

I believe that the best way to handle it is to show the page in the user's computer language and show a pop-up in the language of the country asking the user if they want to switch to the local language.

Of course, an important detail is putting the "stay in <browser language>" button in the browser language.