This is to share my findings on this subject for the benefit of others struggling for a functionality that should be relied on.
In DNN 7.4.2 Browser Language Detection was working like a charm, in DNN 8.01 and 8.0.2 it no longer works.
For example, if in FireFox I set it-IT as preferred language and I try to access www.test.com, DNN redirects to www.text.com/en-us (which is not the expected result).
I found a workaround (for IIS) that works only if the home page has the same name in all languages (i.e. Home) by adding a redirect rule to IIS so that if one writes www.test.com it gets redirected to www.test.com/Home.
In this way, once the preferred language of the browser is set to it-IT, the result is the home page of the site in Italian (although the URL remains www.test.com/Home without the locale in it).
On the contrary, once the preferred language is set to en-US, the result is the home page in English (URL being in this case www.test.com/en-us, how weird!).
I hope this helps while waiting for a definitive fix to this issue.