I have a simple Cocoa project with a Main.storyboard, a Level1.storyboard under a Level1 folder, and a Level2.storyboard in a Level2 subfolder of the Level1 folder. If I Localize those storyboards via Xcode, it creates a structure as shown here.
However, it appears that Xamarin.Mac apps do not support this kind of structured setup of localization files, i.e. it does not pick up the localized strings in those subfolders. Apparently, Xamarin.mac expects all the localized storyboards and all .strings files for them to all exist together in a single set of .lproj folders under "Resources"
In my attached project, I've copied the Level1 storyboard and .strings files up, but not the Level2 ones. please build, run, and observe.
Please confirm if this is a Xamarin.Mac issue.