All the language files go into the assets/minecraft/lang/ folder

Common Languages

en_us	English (US)
en_gb	English (UK)
de_de	Deutsch (Deutschland)
es_es	Español (España)
es_mx	Español (México)
fr_fr	Français (France)
fr_ca	Français (Canada)
it_it	Italiano (Italia)
ja_jp	日本語 (日本)
ko_kr	한국어 (대한민국)
pt_br	Português (Brasil)
pt_pt	Português (Portugal)
ru_ru	Русский (Россия)
zh_cn	简体中文
zh_tw	繁體中文
nl_nl	Nederlands (Nederland)
bg_bg	Български (BG)
cs_cz	Čeština (Česká republika)
da_dk	Dansk (DA)
el_gr	Ελληνικά (Ελλάδα)
fi_fi	Suomi (Suomi)
hu_hu	Magyar (HU)
id_id	Bahasa Indonesia (Indonesia)
nb_no	Norsk bokmål (Norge)
pl_pl	Polski (PL)
sk_sk	Slovensky (SK)
sv_se	Svenska (Sverige)
tr_tr	Türkçe (Türkiye)
uk_ua	Українська (Україна)

All Languages

af_za	Africaans (South Africa)	
ar_sa	Arabic (North Africa & Middle East)
ast_es	Asturian (Asturias)
az_az	Azerbaijani (Azerbaihan)
ba_ru	Bashkir (Bashkortostan Russia)
bar	Bavarian (Bavaria)
be_by	Belarusian (Belarus)
bg_bg	Bulgarian (Bulgaria)
br_fr	Breton (France)
brb	Brabantian (Netherlands)
bs_ba	Bosnian (Bosnia and Herzegovina)
ca_es	Catalan (Spain)
cs_cz	Czech (Czech Republic)
cy_gb	Welsh (Wales)
da_dk	Danish (Denmark)
de_at	Austrian German (Austria)
de_ch	Swiss German (Switzerland)
de_de	German (Germany)
 el_gr	Greek (Greece Cyprus)
en_au	Australian English (Australia)
en_ca	Canadian English (Canada)
en_gb	British English (Great Britain)
en_nz	New Zealand English (New Zealand)
en_pt	Pirate English (The Seven Seas)
en_ud	British English (upside down) Britain
en_us	American English (USA)
enp	English puristic (None)
enws	Early Modern English (None)
eo_uy	Esperanto (Constructed Language)
es_ar	Argentinian Spanish (Argentina)
es_cl	Chilean Spanish (Chile)
es_ec	Ecuadorian Spanish (Ecuador)
es_es	Spanish (Spain)
es_mx	Mexican Spanish (Mexico)
es_uy	Uruguayan Spanish (Uruguay)
es_ve	Venezuelan Spanish (Venezuela)
esan	Andalusian (Andalusia)
et_ee	Estonian (Estonia)
eu_es	Basque (Basque Country)
fa_ir	Persian (Iran)
fi_fi	Finnish (Finland)
fil_ph	Filipino (Philippines)
fo_fo	Faroese (Faroe Islands)
fr_ca	Canadian French (Canada)
fr_fr	French (France)
fra_de	East Franconian (Germany)
fy_nl	Fresian (The Netherlands)
ga_ie	Irish (Ireland)
gd_gb	Scottish Gaelic (Scotland)
gl_es	Galician (Galicia, Spain)
got_de	Gothic (Oium)
gv_im	Manx (Isle of Man)
haw_us	Hawaiian (Hawaii)
he_il	Hebrew (Israel)
hi_in	Hindi (India)
hr_hr	Croatian (Croatia)
hu_hu	Hungarian (Hungary)
hy_am	Armenian (Armenia)
id_id	indonesian (Indonesia)
ig_ng	Igbo (Nigeria)
io_en	Ido (Constructed Language)
is_is	Icelandic (Iceland)
isv	Interslavic (Constructed Language)
it_it	Italian (Italy)
ja_jp	Japanese (Japan)
jbo_en	Lojban (Constructed Language)
ka_ge	Georgian (Georgia)
kab_kab	Kabyle (Algeria)
kk_kz	Kazakh (Kasakhstan)
kn_in	Kannada (India)
ko_kr	Korean (South Korea)
ksh	Kolsch/Ripuarian (Germany)
kw_gb	Cornish (Cornwall)
la_la	Latin (Vatican, Ancient Roman Empire)
lb_lu	Luxembourgish (Luxembourg)
li_li	Limgurgish (The Netherlands)
lol_us	LOLCAT (The Kindgom of Cats)
lt_lt	Lithuanian (Lithuania)
lv_lv	Latvian (Latvia)
mi_nz	Maon (New Zealand)
mk_mk	Macedonian (North Masedonia)
mn_mn	Mongolian (Mongolia)
moh_ca	Mohawk (Canada, United States)
ms_my	Malay (Malaysia)
mt_mt	Maltese (Malta)
nds_de	Low German (Germany)
nl_be	Dutch, Flemish (Belgium)
nl_nl	Dutch (The Netherlands)
nn_no	Norwegian Nynorsk (Norway)
no_no	Norwegian Bokmai (Norway)
nuk	Nu-chah-nulth (Pacific Northwest Coast Canada)
oc_fr	Occitan (Occitania)
oj_ca	Ojibwe (Canada, United States)
ovd	Elfdalian (Alvdalen Sweden)
pl_pl	Polish (Poland)
pt_br	Brazilian Portuguese (Brazil)
pt_pt	Portguese (Portugal)
qya_aa	Quenya (Constructed Language Elvish Lord of the Rings)
ro_ro	Romanian (Romania)
ru_ru	Russian (Russia)
scn	Sicilian (Sicily)
se_no	Northern Sami (Norway)
sk_sk	Slovak (Slovakia)
sl_si	Slovenian (Slovenia)
so_so	Somali (Somalia)
sq_al	Albanian (Albania)
sr_sp	Serbian (Serbia)
sv_se	Swedish (Sweden)
swg	Allgovian German (Germany)
sxu	Upper Saxon German (Saxony)
szl	Silesian (Upper Selisia Poland)
ta_in	Tamil (Sri Lanka)
th_th	Thai (Thailand)
tlh_aa	Klingon (Constructed Language Star Trek)
tr_tr	Turkish (Turkey)
tt_ru	Tatar (Tatarstan Russia)	
tzl_tzl	Talossan (Constructed Language)
uk_ua	Ukrainian (Ukraine)
val_es	Valencian (Spain
vec_it	Venetian (Italy)
vi_vn	Vietnamese (Vietnam)
yi_de	Yiddish (United States)
yo_ng	Yoruba (Nigeria)
zh_cn	Chinese Simplified (China)
zh_tw	Chinese Traditional (Taiwan)