Whether or not embedded bundles should attempt to use the same localization as the main application is controlled by CFBundleAllowMixedLocalizations. Without it, the application tries to display everything uniformly according the top‐level localization. With it, each bundle is given freedom to choose something closer to the user’s preferences instead. Each mode has its tradeoffs.
But either way, CFBundleDevelopmentRegion only comes into play when the available localizations are completely disjoint with the set requested. If an underlying package provides Spanish (as dev), French and German, but the application provides Chinese (as dev), Japanese and Korean, and the user has configured Arabic, Urdu or Farsi, then the development region will cause the package to resolve to Spanish and the application to Chinese.