I have a fresh D8.64+Bootstrap 3 installation, after enable multilingual modules, add languages and config language dection and content/menu translation, I placed the defualt language switcher to the primary sidebar. Then I translated some contents and menus to other languages. Strangely, when I switch languages, nothing happened, even no changes in the admin interface (only the default English)! I changed the theme, changed the language detection order, cleared cache...it was the same. I downloaded a dropdown language block, it even doesn't show up when I place it on the sidebar (but can show in bartik theme). I searched on internet for hours, but no luck. Could anybody have clue of solution?
I finally sort this out：in the language detection and selection set the url first then others. Deleted the languages and reinstalled them. It suddenly worked. I even don't know why. Thank you @jdrupal very much for your patience and help!!