This is the CodeSystem that contains all the codes in ISO 639-1: Codes for the representation of names of languages -- Part 1: Alpha-2 code (http://terminology.hl7.org/CodeSystem/iso639-1) and ISO 639-1: Codes for the representation of names of languages -- Part 1: Alpha-2 code (http://terminology.hl7.org/CodeSystem/iso639-1). E.g. what you have to deal with if you get resources containing codes in either of them