Language resources in Mycroft/OVOS/Neon are organized by BCP-47 Code which is not an official ISO spec (BCP=Best Common Practice) A BCP-47 code is defines as an ISO 639-1 Code (i.e. 'en') and an ISO 3166-1 Alpha-2 Country code (i.e. 'us'). There is no "source of truth" for valid BCP-47 codes; any combination of language and country is valid.
This is an official (outdated) ISO spec for languages. A free list is available on Wikipedia.
This is an official ISO spec for countries; each of the recognized 249 countries has an Alpha-2, Alpha-3, and Numeric code.