[BCP18]    Best Current Practice 18,
              <https://www.rfc-editor.org/info/bcp18>.
              At the time of writing, this BCP comprises the following:

              Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
              January 1998, <https://www.rfc-editor.org/info/rfc2277>.