I was looking for a way to test these localization features without having to resort to changing your Mac’s System Preferences. That seemed like sort of a sledgehammer approach to me. I’m sure it’s fine to do it that way, but I also wanted to point out an alternative that I discovered on Stack Overflow.
You can also go into Xcode, select menu “Product”| “Scheme”| “Edit Scheme…”, then click on the plus under the section “Arguments Passed On Launch” in order to add a command line argument. Then add, as an example, the argument " [color=#0040FF]-AppleLanguages ‘(fr)’[/color] " to test the French version. This suggestion worked for me, and I thought it was probably a much more convenient method if you had a lot of language versions you wished to test. You could probably add multiple schemes, each with a different language argument and manage it that way.