If you create the .app directory using Platypus and do not optimise the .nib file during the process (i.e. deselect Strip nib file to reduce app size), then you can edit the text in Xcode. The important file is in Contents/Resources/MainMenu.nib (see Contents by right-clicking on the .app and selecting Show Package Contents). Then you can right-click on MainMenu.nib and open it in Xcode and edit the text in the Droplet window.