OnSong used to have the ability to change languages directly within the app. However, iOS 12 effectively broke this feature within OnSong, and we were forced to remove it.
This means that OnSong can only be viewed in the language that your iPad or device is set to. So if your device's language from your device's Settings app is in a certain language, OnSong will display that language as well. If your device's language is set to English for example, then OnSong will be in English. To change language in OnSong is to change your iPad's overall language.