How to export a base development language XLIFF file from Xcode 6 and then re-import it to configure additional localizations. (E.g. export en.xliff file and use it to create fr.xliff file if you want to add French to an English development language project)
I am posting this as an "answer your own question" because I spent hours trying to figure this out and as far as I can tell Apple hasn't documented it.
The Apple documentation implies it should be sufficient to rename an exported base language XLIFF with a language prefix to be able to import. I.e. if the base localization is English, you should be able to create a file that you can import for French localization just by renaming it from en.xliff to fr.xliff.
Renaming the file is not enough. You also need to add a target-language attribute to every file element.
From this:
<file original="MyApp/MyApp-Info.plist" source-language="en" datatype="plaintext">
To this:
<file original="MyApp/MyApp-Info.plist" source-language="en" datatype="plaintext" target-language="fr">
Now you can import the file to Xcode and it will create the new localization