Make a difference between system language and application language in iOS

Is it a best practice to make a difference between system language and application language?

For example, you have a phone with English system language, but you want to receive push notifications from, for example, French restaurants list app in French, of course.

But you have trouble ( in iOS ): localizable files swap only in system locale after reboot.

Consider configuration above:

• System language is English.
• French restaurants app language is French.

And a consequences ( or state of phone )

• System language cause French restaurants app to be in English
• French restaurants app language cause your server push notifications to be in French.
• French restaurants notifications as emails are in French too.