Recommended best practice is to use a controlled vocabulary such as RFC 4646 [RFC4646]. (en)
Recommended practice is to use either a non-literal value representing a language from a controlled vocabulary such as ISO 639-2 or ISO 639-3, or a literal value consisting of an IETF Best Current Practice 47 [[IETF-BCP47](https://tools.ietf.org/html/bcp47)] language tag. (en)