Have you accidentally changed your default language on iPhone? And now you are stuck with a language you don’t understand.
I can completely relate to you on this. The first time my iPhone language got changed, I panicked myself. But now knowing how easy it is to change the language on the iPhone, I am not bothered by this change anymore.
Or do you want to change your default language to your regional language on the iPhone? The other case can be that you are not comfortable with the default language of your device. So when Apple has given us the function to choose from a great list of languages, why not make use of it?