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.
 
List of currently supported languages:
Simplified Chinese
English
French
German
Italian
Japanese
Korean
Spanish
Russian
Portuguese
 

Support for per-app language

Set the language of iReal Pro without changing the device's main language.
For example Device language set to German, but iReal Pro showing in Spanish
 
iOS
iOS 13 and higher
Settings/iReal Pro/Language
(If this option does not show it might be because only one language is set in Settings/General/Language & Region, Preferred languages. Add another language.)
 
Android
Android 13 and higher
System Settings/General management/App languages