iReal Pro should automatically recognize the default language of your device.
In case you're having issues, go to your device's Settings/General/Language, set your device's
language to English and reboot it. After that, set it to your language and restart your device.
In iOS 13 and higher, you can set the language of iReal Pro without changing the device's main language in Settings/iReal Pro/Language.
List of currently supported languages: