For the best language bl param, I advice people that are not building a multilingual site to turn this feature off. It gives to possibility to navigate through the site without updating the link in a page
It can't be done automatically as a user can always see a page that is not in his browser preference... You image I want to see the english version , I have to take off my french browser pref to see the page ... not good at all.
bl and browser detection has nothing to do together.
For the browser preference - see batawa he is the author - but I will be happy take some time today to see what I can do
I need to know from which url you have the problem?