In the Xcode's file tree (Project Navigator) select your project. in the right hand pane select your project again. select Info and add your language.
I created a sample project, this is the result:
You can do this directly in the info.plist. Something like this:
<key>CFBundleDevelopmentRegion</key><string>en</string><key>CFBundleLocalizations</key><array> <string>en</string> <string>de</string> <string>es</string> <string>ja</string></array>
Try adding/setting the "Localized resources can be mixed" flag in Info.plist to YES.