By default application interface language coincides with mobile device language. If device language is not supported as an interface language, the first supported language from list of Preferred languages will be selected.
To change order of Preferred languages follow the steps below:
On your mobile device go to Settings > General > Language and region;
Tap Edit in the right upper corner;
Move required language to the first place in the list with languages;
List of supported interface languages is published in application description in App Store.