I localized a part of my application with creating a base.lproj storyboard and 3 string files for it. It was a month ago and after that I added new view controller to app, b
Finally, my solution was (rudimentary but effective) copy the previous translations and paste them in the a new file created by unchecking and checking the Localization language box.
This would be useful in some cases like mine when you have to change only one or two texts.
I think the best way is by using built-in export/import Localizations:
P.S. If you don't want to edit in XLIFF Editor and have skipped step 3, XCode can didn't save new strings. In this case open .xliff in XLIFF Editor, re-save without changes and import new file.