The language that is chosen in the top level domain of the URL.
If it is a domain.es it is in Spanish.
If it is a domain.co.uk it is in English.
If it is a domain.it it is in Italian.
Or just the main language where the website is located. It isn't difficult to add a language icon somewhere at the top or bottom.
Nothing about it needs to track the user. The only info you give them is that you can speak a certain language. Not that it is your main language. Or that you can only understand one.