I've discovered a nice command line option that can be given to any Cocoa Application :
/Applications/toto.app/Content/MacOS/toto -NSShowNonLocalizedStrings YES
Run the application long enough, and you will get all the strings that are not localizable in the application and it's bundles. That a nice trick, no ?
Hence when you want to check if all strings can be localized, you just need to run the application long enough to see if all strings are localizable.