Second Subtag Rules
If the second subtag is:
· Two characters long, it's a country/region defined by ISO 3166
The country codes AA, QM-QZ, XA-XZ and ZZ are reserved by ISO 3166 as user-assigned codes. These must not be used to form language tags.
· Three to eight characters long, it may be registered with the IANA
· One character long, it is illegal
The ISO 3166 country codes are summarized in Table G-3.