ISO 639 assigns a unique two-letter code for most living languages (and
several dead ones).  Here is a full list, sorted alphabetically by code.

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
eo Esperanto
es Spanish
et Estonian
eu Basque
fa Persian
fi Finnish
fj Fiji
fo Faeroese
fr French
fy Frisian
ga Irish
gd 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


Sorted by language name in English:

ab Abkhazian
aa Afar
af Afrikaans
sq Albanian
am Amharic
ar Arabic
hy Armenian
as Assamese
ay Aymara
az Azerbaijani
ba Bashkir
eu Basque
bn Bengali; Bangla
dz Bhutani
bh Bihari
bi Bislama
br Breton
bg Bulgarian
my Burmese
be Byelorussian
km Cambodian
ca Catalan
zh Chinese
co Corsican
hr Croatian
cs Czech
da Danish
nl Dutch
en English
eo Esperanto
et Estonian
fo Faeroese
fj Fiji
fi Finnish
fr French
fy Frisian
gl Galician
ka Georgian
de German
el Greek
kl Greenlandic
gn Guarani
gu Gujarati
ha Hausa
iw Hebrew
hi Hindi
hu Hungarian
is Icelandic
in Indonesian
ia Interlingua
ie Interlingue
ik Inupiak
ga Irish
it Italian
ja Japanese
jw Javanese
kn Kannada
ks Kashmiri
kk Kazakh
rw Kinyarwanda
ky Kirghiz
rn Kirundi
ko Korean
ku Kurdish
lo Laothian
la Latin
lv Latvian, Lettish
ln Lingala
lt Lithuanian
mk Macedonian
mg Malagasy
ms Malay
ml Malayalam
mt Maltese
mi Maori
mr Marathi
mo Moldavian
mn Mongolian
na Nauru
ne Nepali
no Norwegian
oc Occitan
or Oriya
om Oromo (Afan)
ps Pashto, Pushto
fa Persian
pl Polish
pt Portuguese
pa Punjabi
qu Quechua
rm Rhaeto-Romance
ro Romanian
ru Russian
sm Samoan
sg Sangro
sa Sanskrit
gd Scots Gaelic
sr Serbian
sh Serbo-Croatian
st Sesotho
tn Setswana
sn Shona
sd Sindhi
si Singhalese
ss Siswati
sk Slovak
sl Slovenian
so Somali
es Spanish
su Sudanese
sw Swahili
sv Swedish
tl Tagalog
tg Tajik
ta Tamil
tt Tatar
te Tegulu
th Thai
bo Tibetan
ti Tigrinya
to Tonga
ts Tsonga
tr Turkish
tk Turkmen
tw Twi
uk Ukrainian
ur Urdu
uz Uzbek
vi Vietnamese
vo Volapuk
cy Welsh
wo Wolof
xh Xhosa
ji Yiddish
yo Yoruba
zu Zulu