Depending on the app you're using, the method to change its language may change. Some apps are based on the system's language, while others offer their own options for changing the language.
The app is based on your iPhone's language If the app is based on your settings, you need to change the language of your iPhone by going to Settings > General > Language & Region > iPhone Language.
The app offers its own language options In this case, open that app and navigate to its settings to find the language option and change the language used.
The app doesn't change the language If it doesn't change the language when you change the system's language or has no language settings, then that app doesn't support the desired language or is not multilingual.