From Moodle 1.8 on, the default front page language setting is overrided by the browser language configuration. There isn't even mention of this in the help for the page, the release notes or the documentation.
As proposed in
MDL-8791 (http://tracker.moodle.org/browse/MDL-8791#action_27506), I've implemented an option in the language settings page to specify if we prefer the browser settings or the admin configured settings.
The attached patches implement it for 1.8 and 19dev, current as of today.