Craft is supporting multiple languages. You can set the preferred language from the Settings app and from Craft directly as well.

If you are experiencing an issue where the app changes back to the OS default language with every app start you should check if you have multiple languages installed on your device.

To do that, head to Settings > General > Language & Region and ensure you have more than one language on the list. If you don’t, the per-app Language option won’t appear and Craft will switches back to the available installed language.

You can easily solve this issue with installing the needed language. Once it's done the app won't change back the language with every app start.