According to this answer you can run the following code to get the list of languages:
for (Locale locale : Locale.getAvailableLocales()) {
Log.d("LOCALES", locale.getLanguage() + "_" + locale.getCountry() + " [" + locale.getDisplayName() + "]");
}
After running the script and cross referencing it with the results from API23 I got the following:
af_ [Afrikaans]
af_ZA [Afrikaans (South Africa)]
agq_ [Aghem]
am_ET [Amharic (Ethiopia)]
ar_AE [Arabic (United Arab Emirates)]
ar_DZ [Arabic (Algeria)]
ar_IL [Arabic (Israel)]
ar_JO [Arabic (Jordan)]
ar_KM [Arabic (Comoros)]
ar_LY [Arabic (Libya)]
ar_MA [Arabic (Morocco)]
ar_QA [Arabic (Qatar)]
ar_SO [Arabic (Somalia)]
ar_SS [Arabic (South Sudan)]
ar_TN [Arabic (Tunisia)]
asa_ [Asu]
az_ [Azerbaijani (Cyrillic)]
bas_CM [Basaa (Cameroon)]
be_BY [Belarusian (Belarus)]
bem_ [Bemba]
bez_ [Bena]
bg_ [Bulgarian]
bm_ [Bambara (Latin)]
bm_ [Bambara]
bm_ML [Bambara (Latin,Mali)]
bm_ML [Bambara (Mali)]
bn_ [Bengali]
bn_IN [Bengali (India)]
br_FR [Breton (France)]
brx_IN [Bodo (India)]
bs_ [Bosnian (Cyrillic)]
bs_BA [Bosnian (Latin,Bosnia & Herzegovina)]
ca_ [Catalan]
ca_AD [Catalan (Andorra)]
ca_ES [Catalan (Spain)]
ca_FR [Catalan (France)]
ce_ [Chechen]
cgg_UG [Chiga (Uganda)]
cs_CZ [Czech (Czech Republic)]
da_ [Danish]
de_ [German]
de_LU [German (Luxembourg)]
dje_NE [Zarma (Niger)]
dsb_ [Lower Sorbian]
dsb_DE [Lower Sorbian (Germany)]
dua_CM [Duala (Cameroon)]
dz_BT [Dzongkha (Bhutan)]
ee_TG [Ewe (Togo)]
el_GR [Greek (Greece)]
en_AG [English (Antigua & Barbuda)]
en_AI [English (Anguilla)]
en_AT [English (Austria)]
en_BE [English (Belgium)]
en_BM [English (Bermuda)]
en_CA [English (Canada)]
en_CC [English (Cocos (Keeling) Islands)]
en_CH [English (Switzerland)]
en_CK [English (Cook Islands)]
en_CM [English (Cameroon)]
en_CY [English (Cyprus)]
en_ER [English (Eritrea)]
en_FJ [English (Fiji)]
en_FK [English (Falkland Islands (Islas Malvinas))]
en_GM [English (Gambia)]
en_HK [English (Hong Kong)]
en_IE [English (Ireland)]
en_JE [English (Jersey)]
en_JM [English (Jamaica)]
en_KI [English (Kiribati)]
en_KN [English (St. Kitts & Nevis)]
en_LC [English (St. Lucia)]
en_LR [English (Liberia)]
en_MG [English (Madagascar)]
en_MH [English (Marshall Islands)]
en_MS [English (Montserrat)]
en_MT [English (Malta)]
en_NG [English (Nigeria)]
en_NL [English (Netherlands)]
en_NU [English (Niue)]
en_PN [English (Pitcairn Islands)]
en_PR [English (Puerto Rico)]
en_SD [English (Sudan)]
en_SG [English (Singapore)]
en_SL [English (Sierra Leone)]
en_TC [English (Turks & Caicos Islands)]
en_TO [English (Tonga)]
en_UG [English (Uganda)]
en_US [English (United States)]
en_US [English (United States,Computer)]
en_VG [English (British Virgin Islands)]
en_VI [English (U.S. Virgin Islands)]
en_VU [English (Vanuatu)]
en_ZA [English (South Africa)]
eo_ [Esperanto]
es_ [Spanish]
es_419 [Spanish (Latin America)]
es_AR [Spanish (Argentina)]
es_CR [Spanish (Costa Rica)]
es_CU [Spanish (Cuba)]
es_EA [Spanish (Ceuta & Melilla)]
es_EC [Spanish (Ecuador)]
es_GQ [Spanish (Equatorial Guinea)]
es_HN [Spanish (Honduras)]
es_SV [Spanish (El Salvador)]
es_VE [Spanish (Venezuela)]
fr_ [French]
fr_BI [French (Burundi)]
fr_BJ [French (Benin)]
fr_CI [French (Côte d’Ivoire)]
fr_GA [French (Gabon)]
fr_GN [French (Guinea)]
fr_GP [French (Guadeloupe)]
fr_KM [French (Comoros)]
fr_LU [French (Luxembourg)]
fr_PF [French (French Polynesia)]
fr_SC [French (Seychelles)]
fr_SY [French (Syria)]
fr_TD [French (Chad)]
fr_TG [French (Togo)]
fr_TN [French (Tunisia)]
fr_VU [French (Vanuatu)]
fr_WF [French (Wallis & Futuna)]
fur_ [Friulian]
fur_IT [Friulian (Italy)]
fy_NL [Western Frisian (Netherlands)]
ga_ [Irish]
ga_IE [Irish (Ireland)]
gd_ [Scottish Gaelic]
gd_GB [Scottish Gaelic (United Kingdom)]
gl_ [Galician]
gl_ES [Galician (Spain)]
gsw_ [Swiss German]
gsw_CH [Swiss German (Switzerland)]
gsw_FR [Swiss German (France)]
gsw_LI [Swiss German (Liechtenstein)]
gu_ [Gujarati]
gu_IN [Gujarati (India)]
guz_ [Gusii]
guz_KE [Gusii (Kenya)]
gv_ [Manx]
gv_IM [Manx (Isle of Man)]
ha_NG [Hausa (Nigeria)]
haw_ [Hawaiian]
haw_US [Hawaiian (United States)]
hi_IN [Hindi (India)]
hr_BA [Croatian (Bosnia & Herzegovina)]
hsb_ [Upper Sorbian]
hsb_DE [Upper Sorbian (Germany)]
hu_HU [Hungarian (Hungary)]
ig_ [Igbo]
ig_NG [Igbo (Nigeria)]
ii_ [Sichuan Yi]
ii_CN [Sichuan Yi (China)]
is_ [Icelandic]
is_IS [Icelandic (Iceland)]
it_ [Italian]
it_CH [Italian (Switzerland)]
it_IT [Italian (Italy)]
it_SM [Italian (San Marino)]
iw_IL [Hebrew (Israel)]
jgo_ [Ngomba]
jgo_CM [Ngomba (Cameroon)]
ji_ [Yiddish]
ji_001 [Yiddish (World)]
jmc_ [Machame]
jmc_TZ [Machame (Tanzania)]
ka_ [Georgian]
ka_GE [Georgian (Georgia)]
kab_ [Kabyle]
kam_ [Kamba]
kam_KE [Kamba (Kenya)]
kde_ [Makonde]
kea_ [Kabuverdianu]
kea_CV [Kabuverdianu (Cape Verde)]
khq_ [Koyra Chiini]
khq_ML [Koyra Chiini (Mali)]
ki_ [Kikuyu]
ki_KE [Kikuyu (Kenya)]
kkj_ [Kako]
kkj_CM [Kako (Cameroon)]
kl_GL [Kalaallisut (Greenland)]
kln_ [Kalenjin]
km_ [Khmer]
kn_IN [Kannada (India)]
ko_ [Korean]
ko_KP [Korean (North Korea)]
ko_KR [Korean (South Korea)]
kok_IN [Konkani (India)]
ksb_ [Shambala]
kw_ [Cornish]
kw_GB [Cornish (United Kingdom)]
ky_ [Kyrgyz]
lag_TZ [Langi (Tanzania)]
lb_ [Luxembourgish]
lb_LU [Luxembourgish (Luxembourg)]
lg_UG [Ganda (Uganda)]
lkt_ [Lakota]
ln_CD [Lingala (Congo (DRC))]
ln_CF [Lingala (Central African Republic)]
ln_CG [Lingala (Congo (Republic))]
lo_ [Lao]
lu_CD [Luba-Katanga (Congo (DRC))]
luo_KE [Luo (Kenya)]
luy_ [Luyia]
lv_ [Latvian]
lv_LV [Latvian (Latvia)]
mas_ [Masai]
mas_KE [Masai (Kenya)]
mas_TZ [Masai (Tanzania)]
mer_KE [Meru (Kenya)]
mfe_MU [Morisyen (Mauritius)]
mg_ [Malagasy]
mg_MG [Malagasy (Madagascar)]
mgh_MZ [Makhuwa-Meetto (Mozambique)]
mgo_ [Metaʼ]
mgo_CM [Metaʼ (Cameroon)]
mk_MK [Macedonian (Macedonia (FYROM))]
ml_ [Malayalam]
ml_IN [Malayalam (India)]
mn_MN [Mongolian (Mongolia)]
mr_ [Marathi]
mr_IN [Marathi (India)]
ms_ [Malay]
ms_BN [Malay (Brunei)]
ms_MY [Malay (Malaysia)]
mt_ [Maltese]
mt_MT [Maltese (Malta)]
my_ [Burmese]
mzn_ [Mazanderani]
mzn_IR [Mazanderani (Iran)]
naq_NA [Nama (Namibia)]
nb_ [Norwegian Bokmål]
nb_NO [Norwegian Bokmål (Norway)]
nd_ [North Ndebele]
nd_ZW [North Ndebele (Zimbabwe)]
ne_ [Nepali]
ne_IN [Nepali (India)]
ne_NP [Nepali (Nepal)]
nl_ [Dutch]
nl_AW [Dutch (Aruba)]
nl_BE [Dutch (Belgium)]
nl_NL [Dutch (Netherlands)]
nmg_ [Kwasio]
nmg_CM [Kwasio (Cameroon)]
nn_ [Norwegian Nynorsk]
nn_NO [Norwegian Nynorsk (Norway)]
nnh_ [Ngiemboon]
nus_ [Nuer]
nyn_ [Nyankole]
om_ [Oromo]
om_ET [Oromo (Ethiopia)]
om_KE [Oromo (Kenya)]
or_ [Oriya]
os_ [Ossetic]
os_GE [Ossetic (Georgia)]
pa_ [Punjabi (Arabic)]
pa_ [Punjabi]
pa_IN [Punjabi (Gurmukhi,India)]
pa_PK [Punjabi (Arabic,Pakistan)]
pl_PL [Polish (Poland)]
ps_ [Pashto]
pt_AO [Portuguese (Angola)]
pt_BR [Portuguese (Brazil)]
pt_CV [Portuguese (Cape Verde)]
pt_GW [Portuguese (Guinea-Bissau)]
pt_MO [Portuguese (Macau)]
pt_MZ [Portuguese (Mozambique)]
pt_ST [Portuguese (São Tomé & Príncipe)]
pt_TL [Portuguese (Timor-Leste)]
qu_ [Quechua]
qu_BO [Quechua (Bolivia)]
qu_PE [Quechua (Peru)]
rm_CH [Romansh (Switzerland)]
rn_BI [Rundi (Burundi)]
ro_MD [Romanian (Moldova)]
ro_RO [Romanian (Romania)]
ru_BY [Russian (Belarus)]
ru_KG [Russian (Kyrgyzstan)]
ru_MD [Russian (Moldova)]
ru_UA [Russian (Ukraine)]
rw_RW [Kinyarwanda (Rwanda)]
rwk_TZ [Rwa (Tanzania)]
sah_ [Sakha]
saq_ [Samburu]
saq_KE [Samburu (Kenya)]
sbp_ [Sangu]
sbp_TZ [Sangu (Tanzania)]
se_ [Northern Sami]
se_FI [Northern Sami (Finland)]
se_NO [Northern Sami (Norway)]
ses_ [Koyraboro Senni]
ses_ML [Koyraboro Senni (Mali)]
sg_ [Sango]
sg_CF [Sango (Central African Republic)]
shi_ [Tachelhit (Latin)]
shi_ [Tachelhit (Tifinagh)]
shi_ [Tachelhit]
shi_MA [Tachelhit (Latin,Morocco)]
shi_MA [Tachelhit (Tifinagh,Morocco)]
si_ [Sinhala]
si_LK [Sinhala (Sri Lanka)]
sk_ [Slovak]
sl_ [Slovenian]
smn_ [Inari Sami]
sn_ [Shona]
so_ [Somali]
sq_ [Albanian]
sq_AL [Albanian (Albania)]
sq_XK [Albanian (Kosovo)]
sr_ [Serbian (Cyrillic)]
sr_ [Serbian (Latin)]
sr_ [Serbian]
sr_BA [Serbian (Latin,Bosnia & Herzegovina)]
sr_ME [Serbian (Cyrillic,Montenegro)]
sr_RS [Serbian (Cyrillic,Serbia)]
sr_XK [Serbian (Latin,Kosovo)]
sv_SE [Swedish (Sweden)]
sw_CD [Swahili (Congo (DRC))]
sw_KE [Swahili (Kenya)]
sw_UG [Swahili (Uganda)]
ta_IN [Tamil (India)]
ta_LK [Tamil (Sri Lanka)]
ta_MY [Tamil (Malaysia)]
ta_SG [Tamil (Singapore)]
te_ [Telugu]
te_IN [Telugu (India)]
teo_UG [Teso (Uganda)]
ti_ [Tigrinya]
ti_ER [Tigrinya (Eritrea)]
ti_ET [Tigrinya (Ethiopia)]
to_TO [Tongan (Tonga)]
tr_ [Turkish]
tr_CY [Turkish (Cyprus)]
tr_TR [Turkish (Turkey)]
tzm_ [Central Atlas Tamazight]
tzm_MA [Central Atlas Tamazight (Morocco)]
ug_ [Uyghur]
ug_CN [Uyghur (China)]
uk_ [Ukrainian]
ur_ [Urdu]
ur_IN [Urdu (India)]
uz_ [Uzbek (Arabic)]
uz_ [Uzbek (Latin)]
uz_ [Uzbek]
uz_AF [Uzbek (Arabic,Afghanistan)]
uz_UZ [Uzbek (Latin,Uzbekistan)]
vai_ [Vai (Latin)]
vai_ [Vai (Vai)]
vai_LR [Vai (Latin,Liberia)]
vi_ [Vietnamese]
vi_VN [Vietnamese (Vietnam)]
vun_ [Vunjo]
vun_TZ [Vunjo (Tanzania)]
wae_CH [Walser (Switzerland)]
xog_ [Soga]
yav_CM [Yangben (Cameroon)]
yo_ [Yoruba]
yo_NG [Yoruba (Nigeria)]
zgh_ [Standard Moroccan Tamazight]
zgh_MA [Standard Moroccan Tamazight (Morocco)]
zh_ [Chinese (Simplified Han)]
zh_ [Chinese (Traditional Han)]
zh_ [Chinese]
zh_CN [Chinese (Simplified Han,China)]
zh_SG [Chinese (Simplified Han,Singapore)]
zh_TW [Chinese (Traditional Han,Taiwan)]
zu_ [Zulu]
zu_ZA [Zulu (South Africa)]