I have added string localisation according to the instructions found on Android Official Documents. But despite that when I run the app with the langauge & input set prioritising one of then non default languages that have been translated, it doesn't appear in that language on the app.
I have tried 2 physical devices and a virtual one.
I have added the correct files: 
I have set my device through language and input to one of the translated languages: 
All other apps such as google maps then appear in the new language, but, my app still appears in English (my default language).
Every string that is in the default is translated in the other languages.
I am using Android Studio 3.5 Beta 5.
What am I doing wrong?
Check your app's build.gradle file for resConfigs directives that would filter out translations from the resulting APK. All your supported languages should be listed in resConfigs.
(Alternative: remove resConfigs altogether with the cost of getting possibly unused/unsupported resources for dependencies.)
https://developer.android.com/studio/build/shrink-code#unused-alt-resources