ISO 639-3 code
https://en.wikipedia.org/wiki/ISO_639-3
Three letter language codes.
tango.info uses three letter language codes, internally and in URLs. Thus it is tango.info/spa for the Spanish-language site and tango.info/fra for the French-language site.
Two letter codes are reserved for country encoding. Thus tango.info/es, tango.info/fr are for Spain and France.
see also: User:Tobiasco/iso639