This is the ValueSet that contains all the codes in Language codes with language and optionally a region modifier (http://hl7.org/fhir/us/core/ValueSet/simple-language) and Language codes with language and optionally a region modifier (http://hl7.org/fhir/us/core/ValueSet/simple-language). E.g. what you have to deal with if you get resources containing codes in either of them