ISO 639-3 code
http://www.iso.org/iso/catalogue_detail?csnumber=39534
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