Language switching does not work when browser language is configuired in drupal 8.3.7
It is very obvious why this is happening. Language selection process the language detection systems from top to bottom. (Top has highest priority.) Your configuration means that the browser will always be selected first and that url and user settings are ignored. 

If you move browser below user (drag with the arrows icon), the processing order will be:


url
user (if url resolved no language)
browser (if user had no language configured)