This is region designator (ISO 3166-1 standard), as explained here:
https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/LanguageandLocaleIDs/LanguageandLocaleIDs.html#//apple_ref/doc/uid/10000171i-CH15
If you don’t care about user region formatting (e.g. 12hr vs 24hr clock) you can simply use getLanguage method.