In-app strings for different languages are part of the app, and store listing content (short description, long description, and title) is not part of the app.
Both are independent to each other, and both are important. In-app strings for different languages help users access the app in their languages, and thus improves the retention. Store listing content in different languages help users discover the app in an easy manner (How? Default store listing translations are not as good as manual translations for the store listing content).
In short, you do not have to have store listing in multiple languages, but it is strongly recommended to have listing in multi language when you already have in-app multi language support.