This list also appears on pp. 460-462 of Niederst's Web Design in a Nutshell. Major world languages have been highlighted in bold text. (If you disagree with my definition of what makes for a "major world language", my apologies, but no complaints please.)
aa | Afar |
ia | Interlingua |
rn | Kirundi |
ab | Abkhazian |
id | Indonesian |
ro | Romanian |
af | Afrikaans |
ik | Inupiak |
ru | Russian |
ar | Arabic |
is | Icelandic |
sa | Sanskrit |
as | Assamese |
it | Italian |
sd | Sindhi |
ay | Aymara |
iu | Inuktitut |
sg | Sangho |
az | Azerbaijani |
ja | Japanese |
sh | Serbo-Croat |
ba | Bashkir |
jw | Javanese |
si | Sinhalese |
be | Belarussian |
ka | Georgian |
sk | Slovak |
bg | Bulgarian |
kk | Kazakh |
sl | Slovenian |
bh | Bihari |
kl | Greenlandic |
sm | Samoan |
bi | Bislama |
km | Cambodian |
sn | Shona |
bn | Bengali |
kn | Kannada |
so | Somali |
bo | Tibetan |
ko | Korean |
sq | Albanian |
br | Breton |
ks | Kashmiri |
sr | Serbian |
ca | Catalan |
ku | Kurdish |
ss | Siswati |
co | Corsican |
ky | Kyrghiz |
st | Sesotho |
cs | Czech |
la | Latin |
su | Sundanese |
cy | Welsh |
lm | Lingala |
sv | Swedish |
da | Danish |
lo | Laotian |
sw | Swahili |
de | German |
lt | Lithuanian |
ta | Tamil |
dz | Bhutani |
lv | Latvian |
te | Telugu |
el | Greek |
mg | Malagasy |
tg | Tajik |
en | English |
mi | Maori |
th | Thai |
eo | Esperanto |
mk | Macedonian |
ti | Tigrinya |
es | Spanish |
ml | Malayalam |
tk | Turkmen |
et | Estonian |
mn | Mongolian |
tl | Tagalog |
eu | Basque |
mo | Moldovan |
tn | Setswana |
fa | Persian |
mr | Marathi |
to | Tongan |
fi | Finnish |
ms | Malay |
tr | Turkish |
fj | Fijian |
mt | Maltese |
ts | Tsonga |
fo | Faroese |
my | Burmese |
tt | Tatar |
fr | French |
na | Nauru |
tw | Twi |
fy | Frisian |
ne | Nepali |
ug | Uighur |
ga | Irish |
nl | Dutch |
uk | Ukranian |
gd | Scots Gaelic |
no | Norwegian |
ur | Urdu |
gl | Galician |
oc | Occitan |
uz | Uzbek |
gn | Guarani |
om | Oromo |
vi | Vietnamese |
gu | Gujarati |
or | Oriya |
vo | Volapuk |
ha | Hausa |
pa | Punjabi |
wo | Wolof |
he | Hebrew |
pl | Polish |
xh | Xhosa |
hi | Hindi |
ps | Pashtu |
yi | Yiddish |
hr | Croatian |
pt | Portuguese |
yo | Yoruba |
hu | Hungarian |
qu | Quechua |
zh | Chinese |
hy | Armenian |
rm | Rhaeto-Romance |
zu | Zulu |