This is the official list of locale country letter codes:

AA "Afar"
AB "Abkhazian"
AF "Afrikaans"
AM "Amharic"
AR "Arabic"
AS "Assamese"
AY "Aymara"
AZ "Azerbaijani"
BA "Bashkir"
BE "Byelorussian"
BG "Bulgarian"
BH "Bihari"
BI "Bislama"
BN "Bengali" "Bangla"
BO "Tibetan"
BR "Breton"
CA "Catalan"
CO "Corsican"
CS "Czech"
CY "Welsh"
DA "Danish"
DE "German"
DZ "Bhutani"
EL "Greek"
EN "English" "American"
EO "Esperanto"
ES "Spanish"
ET "Estonian"
EU "Basque"
FA "Persian"
FI "Finnish"
FJ "Fiji"
FO "Faeroese"
FR "French"
FY "Frisian"
GA "Irish"
GD "Gaelic" "Scots Gaelic"
GL "Galician"
GN "Guarani"
GU "Gujarati"
HA "Hausa"
HI "Hindi"
HR "Croatian"
HU "Hungarian"
HY "Armenian"
IA "Interlingua"
IE "Interlingue"
IK "Inupiak"
IN "Indonesian"
IS "Icelandic"
IT "Italian"
IW "Hebrew"
JA "Japanese"
JI "Yiddish"
JW "Javanese"
KA "Georgian"
KK "Kazakh"
KL "Greenlandic"
KM "Cambodian"
KN "Kannada"
KO "Korean"
KS "Kashmiri"
KU "Kurdish"
KY "Kirghiz"
LA "Latin"
LN "Lingala"
LO "Laothian"
LT "Lithuanian"
LV "Latvian" "Lettish"
MG "Malagasy"
MI "Maori"
MK "Macedonian"
ML "Malayalam"
MN "Mongolian"
MO "Moldavian"
MR "Marathi"
MS "Malay"
MT "Maltese"
MY "Burmese"
NA "Nauru"
NE "Nepali"
NL "Dutch"



NO "Norwegian"
OC "Occitan"
OM "Oromo" "Afan"
OR "Oriya"
PA "Punjabi"
PL "Polish"
PS "Pashto" "Pushto"
PT "Portuguese"
QU "Quechua"
RM "Rhaeto-Romance"
RN "Kirundi"
RO "Romanian"
RU "Russian"
RW "Kinyarwanda"
SA "Sanskrit"
SD "Sindhi"
SG "Sangro"
SH "Serbo-Croatian"
SI "Singhalese"
SK "Slovak"
SL "Slovenian"
SM "Samoan"
SN "Shona"
SO "Somali"
SQ "Albanian"
SR "Serbian"
SS "Siswati"
ST "Sesotho"
SU "Sudanese"
SV "Swedish"
SW "Swahili"
TA "Tamil"
TE "Tegulu"
TG "Tajik"
TH "Thai"
TI "Tigrinya"
TK "Turkmen"
TL "Tagalog"
TN "Setswana"
TO "Tonga"
TR "Turkish"
TS "Tsonga"
TT "Tatar"
TW "Twi"
UK "Ukrainian"
UR "Urdu"
UZ "Uzbek"
VI "Vietnamese"
VO "Volapuk"
WO "Wolof"
XH "Xhosa"
YO "Yoruba"
ZH "Chinese"
ZU "Zulu"

Source and information about this can be found in: http://www.loc.gov/standards/iso639-2/php/code_list.php