From the CFBundle documentation:
kCFBundleDevelopmentRegionKey
The name of the development language of the bundle.
When CFBundle looks for resources, the fallback is to look in the lproj whose name is given by the kCFBundleDevelopmentRegionKey in the Info.plist file. You must, therefore, ensure that a bundle contains an lproj with that exact name containing a copy of every localized resource, otherwise CFBundle cannot guarantee the fallback mechanism will work.
Available in iOS 2.0 and later.
Declared in CFBundle.h.
No relation to the AppStore Description language.