FHIR recommendation: Internet Engineering Task Force (IETF) in Best Current Practice (BCP) 47 (i.e. BCP-47 code)
Language tag syntax is defined by the IETF's BCP 47. BCP stands for 'Best Current Practice', and is a persistent name for a series of RFCs whose numbers change as they are updated. The latest RFC describing language tag syntax is RFC 5646, Tags for the Identification of Languages, and it obsoletes the older RFCs 4646, 3066 and 1766.
More details on : https://www.w3.org/International/articles/language-tags/ and https://www.w3.org/International/questions/qa-choosing-language-tags