It'll work same way browsers know what language to present their main content in: either via the Accept-Language HTTP header, or through a UI element on the website that lets the visitor choose the language.
But then they are not based on the context of the website. This is just the same tracking with another name.
>or through a UI element on the website that lets the visitor choose the language.
This is nonsensical too. People don't just speak only one language. If I'm going into an English website and you're giving me a language pop up I'm going to pick English. But ads that are in English are not relevant to me whatsoever.
Also, I would like to note that language redirects are, in my experience, absolute trash. It makes using sites like Adobe awful.
Even the language settings in windows lead to a bad user experience. I have to keep English as my first keyboard language and locale to make sure that websites don't default to other languages for me.
