The language of the AirVisual app is dependent on the language of the phone.
To change the language for your iOS (Apple) phone: Go to the iPhone's settings app (the one that looks like a gear) > Scroll down to "Language & Region" > Select a new iPhone language and confirm > Re-enter the AirVisual App - your app language will have changed.
To change the language for your Android phone: Go to Settings > General management > Language and input > Language > Select your preferred default language to be at the top of the list > Re-enter the AirVisual App - your app language will have changed.