How to localize a Cocoa project ?

I try to copy - at the Finder level - the original English.lproj to a new one, but it doesn't work: I got errors when launching the app… ?

.lproj you want to use as a localization base from inside ProjectBuilder:
Select the original
.lproj in the file list of your project, right-click on it to show the contextual menu and select "Show Info", (Command-I shortcut),
There is a "Localization & platforms" pull-down menu, select "Add localized variant…" (or "Make localized" if the file is not already in localized format), enter the name of the language when prompted for…
PB does some clean-up inside the files when copying them, something the Finder doesn't do: here is the source of the problems when working at the Finder level.
You have to "copy" the original