I saw another Topic here, and have the very same problem about missing language setting in Delta Chat system settings.
The issue occur in China mainland version iPhone, iOS 15.3.1. In “system settings” it even has no “PREFERRED LANGUAGE” setting. But in US version iPhone, the language setting can be displayed normally.
I try to switch Country/Region in App store and reinstall, it won’t help.
I wonder whether it’s a bug for China mainland version iPhone or DeltaChat itself?