The mobile app runs in the system language, so if you want to have the app run in English then your system language must be in English.
Having said that: Apple iOS 13 now allows you to set a different language per app (for some apps), and Lightroom is among those apps. Go to Settings and then scroll down to Lightroom. Tap on it to get to the app settings. Preferred Language is at the bottom. If English is not among them, then you first need to set that as extra language in Settings - General - Language & Region.