If a user has set their browser to an interface language setting then we can be fairly sure that they need the forum interface language in this language as well. Unless otherwise set in account settings, the user/member/guest should see the site in the same language as their browser.
I was searching for an add-on that does this, but instead I only found this suggestion. I'm guessing it hasn't been implemented, neither officially nor as an add-on? It seems really simple to implement.