Language switcher is not working

I could see the language switcher in main menu. But when I choose the secondary language Arabic nothing happened. It was still remains in primary language. When I change the url manually that is mysite.com/ar its working fine.
I am using WordPress default theme Twenty Sixteen.

I have tried of disabling all the plugins and testing with all the other default themes. But it’s the same.

I am bad with php. Any kind of help to fix this will be appreciated ..
Thanks