Don’t worry, we can help you!  First of all, it’s important to know that your iPhone tries to guess the best fitting language for any app on your device. Naturally, the first guess is your phone’s language. But if an app is not available in that language, then your phone will work its way through a list of languages, and use the first language that is also supported by the app. Fortunately, you can edit this list:
Open your iPhone’s ‘Settings’
Tap on ‘General’
Select ‘Language & Region’
Check if the ‘Preferred language order’ is correct for you.
In this example, you iPhone will try to use English first, then French, then Spanish, then Japanese etc.
If the order is not right for you, then you can tap on ‘Edit’ to change the order or delete a language. If you select  ‘Add Language…”, you can also add a new language to this list.
Please note, that the first language in this list is also your iPhone’s default language.
But wait, there is more! 
Starting with iOS 13, you can set or change the language for a single app as well: Open ‘Settings’, scroll down and select the app you would like to set the language for – e.g. Moodistory. Then you just have to tap on ‘Language’ to change the language.