[BCP47]    Best Current Practice 47,
              <https://www.rfc-editor.org/info/bcp47>.
              At the time of writing, this BCP comprises the following:

              Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
              Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September
              2006, <https://www.rfc-editor.org/info/rfc4647>.

              Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.