IANA Language Subtag Registry
https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

The lookup in registry.json is derived from the IANA Language Subtag Registry (RFC 5646).
The IANA/IETF joint statement on the Protocol Registries (https://www.iana.org/help/licensing-terms) applies:

  IANA and IETF intend that the Protocol Registries may be freely used by any party for any purpose.
  Any applicable rights that they may have in the Protocol Registries are subject to the
  Creative Commons CC0 1.0 dedication (https://creativecommons.org/publicdomain/zero/1.0/legalcode).
  The Protocol Registries are made available without any other sort of representation or warranty.
