I am trying to build an in-app language switcher for the app I am working on, but I have tried almost everything method available on Stackoverflow. I also tried Official guide from here 
Another thing,In Your case go to Settings Screen DropDown menu, Once select any language you have to store that code not name
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More Info : if we use AppCompatDelegate.setApplicationLocales() with MaterialTheme, Context is becomes NULL, so its not able to create Locale Manager.
the Context is null because of sActivityDelegates is empty.
Even I don't know what is sActivityDelegates
But, as soon as i change the Theme to AppCompat. its becomes working.
Tell us, if someone know about sActivityDelegates, and why its not adding when lifecycle in postCreate as they mention in Comments. it would be appreciated.