How language based redirection works

I have started setting up new language versions and have not yet added a language switcher to frontend or workflows.
Some of my users whose device language matches my new language started sending error “Not found errors. Redirecting to …/404”. I was able to trace that the last page the users visited was the new language page.
How does such a redirect work if I did not add workflows and language switches? Is there a binding to the user’s device language?

UPD: 404 redirect doesn’t work