I can’t remember one feature being mentioned on stage during the iOS 13 introduction at WWDC: you can control the language of apps on iOS 13 individually in Settings.app, provided that the app itself supports more than one language.
While I strongly prefer to use English as the primary language on operating system level1, there are some apps that I always wished I could run in my native German language.
But unfortunately, this was not possible in iOS 12 and before. Unless an app itself offered the ability to switch to a different language (and German was a choice) you were out of luck.
But again, in some cases it makes much more sense to run an app in German. A prominent example is the DB Navigator2 app by Deutsche Bahn, or any other public transport app of any German-speaking city that I regularly travel to. It just feels more … natural.
I’m very happy about this unheralded little nugget. It will certainly improve my iOS experience a lot.