[BCP58]    Best Current Practice 58,
              <https://www.rfc-editor.org/info/bcp58>.
              At the time of writing, this BCP comprises the following:

              Hoffman, P. and S. Bradner, "Defining the IETF", BCP 58,
              RFC 3233, DOI 10.17487/RFC3233, February 2002,
              <https://www.rfc-editor.org/info/rfc3233>.