language selection in-app are quite unusual on ios, the system just choses the “correct” language to be used for the app. and when there is more than one defined, the user can define a language in the system settings. afaik, eg. android does not have such a setting, therefore, it is usually done in-app there. adding such a setting to ios, however, would probably only add more confusion as there are easily conflicts.
as this is the same for all apps, this is quite comfortable for ios users. however, obviously, may be irritating for non-ios users
so, remaining question would be if really “Chinese Simplified” is not selected when the system is set to “Chinese Simplified”, that needs to be double checked, at best on another device. might be there is a bug in apples language selection, esp. if the selection works when selection is done manually.