I have just one app not in English. How to change?

Then you’ll need to contact the developer of the app. it was likely poorly coded to ignore the language settings of iOS.

Go the the Settings for that app and see if there is a place to choose the language.

You can try contacting the developer. The information in the link below may be helpful. If not, there is nothing else you can do. It is up to the developer to decide which languages to support.

How to contact an app developer - Apple Support