Internationalization Q&A

Hi, my apps are build the way, that it always should take the localization based on the system language. I had/have the issue, that sometimes it falls back to english. I assume, that the configured user language isn't fully localized. you can see that especially when using date and time formats and calendars. how do I prevent that? or: how is the correct way to implement localization in an app? thanks in advance

Are these two things the same? If not, how are they different?