Unfortunately, I don't think you can.
iOS has no way to tell what language preference is set inside each application (well, it could, but it's not done that way).
Instead, it uses the system-wide setting to find and display the appropriate CFBundleDisplayName
for every application.
Developing a custom language preference inside an app is also not the recommended way. Apple expects users to set their language of choice in the device's Preferences, not from inside individual apps (see here: https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPInternational/Articles/InternatSupport.html#//apple_ref/doc/uid/20000278-SW1). As an example, there's no way to change Facebook's language from inside the app, but as soon as you change the device's global language setting, the Facebook app reflects the change.