Localization problems when migrating to Xcode 4

French.lproj is the old way and fr.lproj is the new, recommended way. I'm not surprised you are having mixed success just copying and pasting as I've also tried that with limited success. It shouldn't be much difficulty to re-add the localizations assuming you save a copy of your existing ones. Then you can just use the finder to copy your localized files over the new XCode generated ones. I would think that would work.

I will give you a heads up and hopefully you have more luck than me. I had incredibly crashing problems with localizing images in XCode 4. I would click to add localizations and XCode would frequently crash. It would copy the file to the en.lproj but none others. Then upon re-open if I selected the messed up localized file it would crash again. I eventually got stuck in a crash cycle and loaded it up in XCode 3 to create the localizations for a couple images and then was able to get it to load properly in XCode 4.

Oh, and I assume you did a clean and build.

 

Did you try AppleGlot?

You might want to try AppleGlot to update your old localized xibs/nibs. You should populate the "_NewBase" folder with your new Xcode 4 English files, the "OldBase" folder with your previous Xcode 3 English files, and finally the "_OldLoc" folder with your old French translations. Most (if not all) the translations should be automatically leveraged. Let me know if that works.
Best regards,
Eduard