Manually I just go into the system preferences, change the language ordering, and restart my app so it starts up in the default system language. Then I can run automated tests with the app running in that language.
In the command line, there is a built-in OSX utility languagesetup but this requires sudo and it has to prompt for the new language (i.e. there is no obvious command line option).
I don't know AppleScript well enough to use that but I would be open to using it.