Android: how should an app handle the device's language change

Actually I'm handling 3 languages in my app so I have:

values/strings.xml

values-it/strings.xml

values-de/strings.xml

When I install the app, it is properly installed in the current language of the device. But if I change the language of the device, the language of the app doesn't switch. I have to reinstall it to switch the language.

I've seent that this happens also to other apps, but not to all. The "big" ones (whatsapp, viber, all google's, etc) change.

So the question is: how can an app follow the device's language change ?