A resposta aceita falta de alguns locais bastante significativos. Por exemplo, as localidades chinesas não fazem distinção entre as diferentes escrituras (han simplificado, han tradicional). O que é um cidadão chinês que quer o han tradicional deveria usar - o cenário de Hong Kong?
Peguei essa lista de códigos de idioma do iOS , removi a entrada POSIX estranha, formatou-a como Javascript e a classifiquei por valor (por isso, se você usar uma implementação de objeto ordenada e criar uma lista percorrendo as chaves e retornando o valor, ele será mostrar o texto legível por humanos na ordem correta). Além disso, esta lista contém uma entrada principal por idioma e todas as variantes específicas da região - dessa forma, você sempre tem uma opção de fallback designada, digamos que se sua interface definiu apenas a opção 'en' - você não saberia no que recorrer de forma confiável.
var LANGUAGE_BY_LOCALE = {
af_NA: "Afrikaans (Namibia)",
af_ZA: "Afrikaans (South Africa)",
af: "Afrikaans",
ak_GH: "Akan (Ghana)",
ak: "Akan",
sq_AL: "Albanian (Albania)",
sq: "Albanian",
am_ET: "Amharic (Ethiopia)",
am: "Amharic",
ar_DZ: "Arabic (Algeria)",
ar_BH: "Arabic (Bahrain)",
ar_EG: "Arabic (Egypt)",
ar_IQ: "Arabic (Iraq)",
ar_JO: "Arabic (Jordan)",
ar_KW: "Arabic (Kuwait)",
ar_LB: "Arabic (Lebanon)",
ar_LY: "Arabic (Libya)",
ar_MA: "Arabic (Morocco)",
ar_OM: "Arabic (Oman)",
ar_QA: "Arabic (Qatar)",
ar_SA: "Arabic (Saudi Arabia)",
ar_SD: "Arabic (Sudan)",
ar_SY: "Arabic (Syria)",
ar_TN: "Arabic (Tunisia)",
ar_AE: "Arabic (United Arab Emirates)",
ar_YE: "Arabic (Yemen)",
ar: "Arabic",
hy_AM: "Armenian (Armenia)",
hy: "Armenian",
as_IN: "Assamese (India)",
as: "Assamese",
asa_TZ: "Asu (Tanzania)",
asa: "Asu",
az_Cyrl: "Azerbaijani (Cyrillic)",
az_Cyrl_AZ: "Azerbaijani (Cyrillic, Azerbaijan)",
az_Latn: "Azerbaijani (Latin)",
az_Latn_AZ: "Azerbaijani (Latin, Azerbaijan)",
az: "Azerbaijani",
bm_ML: "Bambara (Mali)",
bm: "Bambara",
eu_ES: "Basque (Spain)",
eu: "Basque",
be_BY: "Belarusian (Belarus)",
be: "Belarusian",
bem_ZM: "Bemba (Zambia)",
bem: "Bemba",
bez_TZ: "Bena (Tanzania)",
bez: "Bena",
bn_BD: "Bengali (Bangladesh)",
bn_IN: "Bengali (India)",
bn: "Bengali",
bs_BA: "Bosnian (Bosnia and Herzegovina)",
bs: "Bosnian",
bg_BG: "Bulgarian (Bulgaria)",
bg: "Bulgarian",
my_MM: "Burmese (Myanmar [Burma])",
my: "Burmese",
yue_Hant_HK: "Cantonese (Traditional, Hong Kong SAR China)",
ca_ES: "Catalan (Spain)",
ca: "Catalan",
tzm_Latn: "Central Morocco Tamazight (Latin)",
tzm_Latn_MA: "Central Morocco Tamazight (Latin, Morocco)",
tzm: "Central Morocco Tamazight",
chr_US: "Cherokee (United States)",
chr: "Cherokee",
cgg_UG: "Chiga (Uganda)",
cgg: "Chiga",
zh_Hans: "Chinese (Simplified Han)",
zh_Hans_CN: "Chinese (Simplified Han, China)",
zh_Hans_HK: "Chinese (Simplified Han, Hong Kong SAR China)",
zh_Hans_MO: "Chinese (Simplified Han, Macau SAR China)",
zh_Hans_SG: "Chinese (Simplified Han, Singapore)",
zh_Hant: "Chinese (Traditional Han)",
zh_Hant_HK: "Chinese (Traditional Han, Hong Kong SAR China)",
zh_Hant_MO: "Chinese (Traditional Han, Macau SAR China)",
zh_Hant_TW: "Chinese (Traditional Han, Taiwan)",
zh: "Chinese",
kw_GB: "Cornish (United Kingdom)",
kw: "Cornish",
hr_HR: "Croatian (Croatia)",
hr: "Croatian",
cs_CZ: "Czech (Czech Republic)",
cs: "Czech",
da_DK: "Danish (Denmark)",
da: "Danish",
nl_BE: "Dutch (Belgium)",
nl_NL: "Dutch (Netherlands)",
nl: "Dutch",
ebu_KE: "Embu (Kenya)",
ebu: "Embu",
en_AS: "English (American Samoa)",
en_AU: "English (Australia)",
en_BE: "English (Belgium)",
en_BZ: "English (Belize)",
en_BW: "English (Botswana)",
en_CA: "English (Canada)",
en_GU: "English (Guam)",
en_HK: "English (Hong Kong SAR China)",
en_IN: "English (India)",
en_IE: "English (Ireland)",
en_IL: "English (Israel)",
en_JM: "English (Jamaica)",
en_MT: "English (Malta)",
en_MH: "English (Marshall Islands)",
en_MU: "English (Mauritius)",
en_NA: "English (Namibia)",
en_NZ: "English (New Zealand)",
en_MP: "English (Northern Mariana Islands)",
en_PK: "English (Pakistan)",
en_PH: "English (Philippines)",
en_SG: "English (Singapore)",
en_ZA: "English (South Africa)",
en_TT: "English (Trinidad and Tobago)",
en_UM: "English (U.S. Minor Outlying Islands)",
en_VI: "English (U.S. Virgin Islands)",
en_GB: "English (United Kingdom)",
en_US: "English (United States)",
en_ZW: "English (Zimbabwe)",
en: "English",
eo: "Esperanto",
et_EE: "Estonian (Estonia)",
et: "Estonian",
ee_GH: "Ewe (Ghana)",
ee_TG: "Ewe (Togo)",
ee: "Ewe",
fo_FO: "Faroese (Faroe Islands)",
fo: "Faroese",
fil_PH: "Filipino (Philippines)",
fil: "Filipino",
fi_FI: "Finnish (Finland)",
fi: "Finnish",
fr_BE: "French (Belgium)",
fr_BJ: "French (Benin)",
fr_BF: "French (Burkina Faso)",
fr_BI: "French (Burundi)",
fr_CM: "French (Cameroon)",
fr_CA: "French (Canada)",
fr_CF: "French (Central African Republic)",
fr_TD: "French (Chad)",
fr_KM: "French (Comoros)",
fr_CG: "French (Congo - Brazzaville)",
fr_CD: "French (Congo - Kinshasa)",
fr_CI: "French (Côte d’Ivoire)",
fr_DJ: "French (Djibouti)",
fr_GQ: "French (Equatorial Guinea)",
fr_FR: "French (France)",
fr_GA: "French (Gabon)",
fr_GP: "French (Guadeloupe)",
fr_GN: "French (Guinea)",
fr_LU: "French (Luxembourg)",
fr_MG: "French (Madagascar)",
fr_ML: "French (Mali)",
fr_MQ: "French (Martinique)",
fr_MC: "French (Monaco)",
fr_NE: "French (Niger)",
fr_RW: "French (Rwanda)",
fr_RE: "French (Réunion)",
fr_BL: "French (Saint Barthélemy)",
fr_MF: "French (Saint Martin)",
fr_SN: "French (Senegal)",
fr_CH: "French (Switzerland)",
fr_TG: "French (Togo)",
fr: "French",
ff_SN: "Fulah (Senegal)",
ff: "Fulah",
gl_ES: "Galician (Spain)",
gl: "Galician",
lg_UG: "Ganda (Uganda)",
lg: "Ganda",
ka_GE: "Georgian (Georgia)",
ka: "Georgian",
de_AT: "German (Austria)",
de_BE: "German (Belgium)",
de_DE: "German (Germany)",
de_LI: "German (Liechtenstein)",
de_LU: "German (Luxembourg)",
de_CH: "German (Switzerland)",
de: "German",
el_CY: "Greek (Cyprus)",
el_GR: "Greek (Greece)",
el: "Greek",
gu_IN: "Gujarati (India)",
gu: "Gujarati",
guz_KE: "Gusii (Kenya)",
guz: "Gusii",
ha_Latn: "Hausa (Latin)",
ha_Latn_GH: "Hausa (Latin, Ghana)",
ha_Latn_NE: "Hausa (Latin, Niger)",
ha_Latn_NG: "Hausa (Latin, Nigeria)",
ha: "Hausa",
haw_US: "Hawaiian (United States)",
haw: "Hawaiian",
he_IL: "Hebrew (Israel)",
he: "Hebrew",
hi_IN: "Hindi (India)",
hi: "Hindi",
hu_HU: "Hungarian (Hungary)",
hu: "Hungarian",
is_IS: "Icelandic (Iceland)",
is: "Icelandic",
ig_NG: "Igbo (Nigeria)",
ig: "Igbo",
id_ID: "Indonesian (Indonesia)",
id: "Indonesian",
ga_IE: "Irish (Ireland)",
ga: "Irish",
it_IT: "Italian (Italy)",
it_CH: "Italian (Switzerland)",
it: "Italian",
ja_JP: "Japanese (Japan)",
ja: "Japanese",
kea_CV: "Kabuverdianu (Cape Verde)",
kea: "Kabuverdianu",
kab_DZ: "Kabyle (Algeria)",
kab: "Kabyle",
kl_GL: "Kalaallisut (Greenland)",
kl: "Kalaallisut",
kln_KE: "Kalenjin (Kenya)",
kln: "Kalenjin",
kam_KE: "Kamba (Kenya)",
kam: "Kamba",
kn_IN: "Kannada (India)",
kn: "Kannada",
kk_Cyrl: "Kazakh (Cyrillic)",
kk_Cyrl_KZ: "Kazakh (Cyrillic, Kazakhstan)",
kk: "Kazakh",
km_KH: "Khmer (Cambodia)",
km: "Khmer",
ki_KE: "Kikuyu (Kenya)",
ki: "Kikuyu",
rw_RW: "Kinyarwanda (Rwanda)",
rw: "Kinyarwanda",
kok_IN: "Konkani (India)",
kok: "Konkani",
ko_KR: "Korean (South Korea)",
ko: "Korean",
khq_ML: "Koyra Chiini (Mali)",
khq: "Koyra Chiini",
ses_ML: "Koyraboro Senni (Mali)",
ses: "Koyraboro Senni",
lag_TZ: "Langi (Tanzania)",
lag: "Langi",
lv_LV: "Latvian (Latvia)",
lv: "Latvian",
lt_LT: "Lithuanian (Lithuania)",
lt: "Lithuanian",
luo_KE: "Luo (Kenya)",
luo: "Luo",
luy_KE: "Luyia (Kenya)",
luy: "Luyia",
mk_MK: "Macedonian (Macedonia)",
mk: "Macedonian",
jmc_TZ: "Machame (Tanzania)",
jmc: "Machame",
kde_TZ: "Makonde (Tanzania)",
kde: "Makonde",
mg_MG: "Malagasy (Madagascar)",
mg: "Malagasy",
ms_BN: "Malay (Brunei)",
ms_MY: "Malay (Malaysia)",
ms: "Malay",
ml_IN: "Malayalam (India)",
ml: "Malayalam",
mt_MT: "Maltese (Malta)",
mt: "Maltese",
gv_GB: "Manx (United Kingdom)",
gv: "Manx",
mr_IN: "Marathi (India)",
mr: "Marathi",
mas_KE: "Masai (Kenya)",
mas_TZ: "Masai (Tanzania)",
mas: "Masai",
mer_KE: "Meru (Kenya)",
mer: "Meru",
mfe_MU: "Morisyen (Mauritius)",
mfe: "Morisyen",
naq_NA: "Nama (Namibia)",
naq: "Nama",
ne_IN: "Nepali (India)",
ne_NP: "Nepali (Nepal)",
ne: "Nepali",
nd_ZW: "North Ndebele (Zimbabwe)",
nd: "North Ndebele",
nb_NO: "Norwegian Bokmål (Norway)",
nb: "Norwegian Bokmål",
nn_NO: "Norwegian Nynorsk (Norway)",
nn: "Norwegian Nynorsk",
nyn_UG: "Nyankole (Uganda)",
nyn: "Nyankole",
or_IN: "Oriya (India)",
or: "Oriya",
om_ET: "Oromo (Ethiopia)",
om_KE: "Oromo (Kenya)",
om: "Oromo",
ps_AF: "Pashto (Afghanistan)",
ps: "Pashto",
fa_AF: "Persian (Afghanistan)",
fa_IR: "Persian (Iran)",
fa: "Persian",
pl_PL: "Polish (Poland)",
pl: "Polish",
pt_BR: "Portuguese (Brazil)",
pt_GW: "Portuguese (Guinea-Bissau)",
pt_MZ: "Portuguese (Mozambique)",
pt_PT: "Portuguese (Portugal)",
pt: "Portuguese",
pa_Arab: "Punjabi (Arabic)",
pa_Arab_PK: "Punjabi (Arabic, Pakistan)",
pa_Guru: "Punjabi (Gurmukhi)",
pa_Guru_IN: "Punjabi (Gurmukhi, India)",
pa: "Punjabi",
ro_MD: "Romanian (Moldova)",
ro_RO: "Romanian (Romania)",
ro: "Romanian",
rm_CH: "Romansh (Switzerland)",
rm: "Romansh",
rof_TZ: "Rombo (Tanzania)",
rof: "Rombo",
ru_MD: "Russian (Moldova)",
ru_RU: "Russian (Russia)",
ru_UA: "Russian (Ukraine)",
ru: "Russian",
rwk_TZ: "Rwa (Tanzania)",
rwk: "Rwa",
saq_KE: "Samburu (Kenya)",
saq: "Samburu",
sg_CF: "Sango (Central African Republic)",
sg: "Sango",
seh_MZ: "Sena (Mozambique)",
seh: "Sena",
sr_Cyrl: "Serbian (Cyrillic)",
sr_Cyrl_BA: "Serbian (Cyrillic, Bosnia and Herzegovina)",
sr_Cyrl_ME: "Serbian (Cyrillic, Montenegro)",
sr_Cyrl_RS: "Serbian (Cyrillic, Serbia)",
sr_Latn: "Serbian (Latin)",
sr_Latn_BA: "Serbian (Latin, Bosnia and Herzegovina)",
sr_Latn_ME: "Serbian (Latin, Montenegro)",
sr_Latn_RS: "Serbian (Latin, Serbia)",
sr: "Serbian",
sn_ZW: "Shona (Zimbabwe)",
sn: "Shona",
ii_CN: "Sichuan Yi (China)",
ii: "Sichuan Yi",
si_LK: "Sinhala (Sri Lanka)",
si: "Sinhala",
sk_SK: "Slovak (Slovakia)",
sk: "Slovak",
sl_SI: "Slovenian (Slovenia)",
sl: "Slovenian",
xog_UG: "Soga (Uganda)",
xog: "Soga",
so_DJ: "Somali (Djibouti)",
so_ET: "Somali (Ethiopia)",
so_KE: "Somali (Kenya)",
so_SO: "Somali (Somalia)",
so: "Somali",
es_AR: "Spanish (Argentina)",
es_BO: "Spanish (Bolivia)",
es_CL: "Spanish (Chile)",
es_CO: "Spanish (Colombia)",
es_CR: "Spanish (Costa Rica)",
es_DO: "Spanish (Dominican Republic)",
es_EC: "Spanish (Ecuador)",
es_SV: "Spanish (El Salvador)",
es_GQ: "Spanish (Equatorial Guinea)",
es_GT: "Spanish (Guatemala)",
es_HN: "Spanish (Honduras)",
es_419: "Spanish (Latin America)",
es_MX: "Spanish (Mexico)",
es_NI: "Spanish (Nicaragua)",
es_PA: "Spanish (Panama)",
es_PY: "Spanish (Paraguay)",
es_PE: "Spanish (Peru)",
es_PR: "Spanish (Puerto Rico)",
es_ES: "Spanish (Spain)",
es_US: "Spanish (United States)",
es_UY: "Spanish (Uruguay)",
es_VE: "Spanish (Venezuela)",
es: "Spanish",
sw_KE: "Swahili (Kenya)",
sw_TZ: "Swahili (Tanzania)",
sw: "Swahili",
sv_FI: "Swedish (Finland)",
sv_SE: "Swedish (Sweden)",
sv: "Swedish",
gsw_CH: "Swiss German (Switzerland)",
gsw: "Swiss German",
shi_Latn: "Tachelhit (Latin)",
shi_Latn_MA: "Tachelhit (Latin, Morocco)",
shi_Tfng: "Tachelhit (Tifinagh)",
shi_Tfng_MA: "Tachelhit (Tifinagh, Morocco)",
shi: "Tachelhit",
dav_KE: "Taita (Kenya)",
dav: "Taita",
ta_IN: "Tamil (India)",
ta_LK: "Tamil (Sri Lanka)",
ta: "Tamil",
te_IN: "Telugu (India)",
te: "Telugu",
teo_KE: "Teso (Kenya)",
teo_UG: "Teso (Uganda)",
teo: "Teso",
th_TH: "Thai (Thailand)",
th: "Thai",
bo_CN: "Tibetan (China)",
bo_IN: "Tibetan (India)",
bo: "Tibetan",
ti_ER: "Tigrinya (Eritrea)",
ti_ET: "Tigrinya (Ethiopia)",
ti: "Tigrinya",
to_TO: "Tonga (Tonga)",
to: "Tonga",
tr_TR: "Turkish (Turkey)",
tr: "Turkish",
uk_UA: "Ukrainian (Ukraine)",
uk: "Ukrainian",
ur_IN: "Urdu (India)",
ur_PK: "Urdu (Pakistan)",
ur: "Urdu",
uz_Arab: "Uzbek (Arabic)",
uz_Arab_AF: "Uzbek (Arabic, Afghanistan)",
uz_Cyrl: "Uzbek (Cyrillic)",
uz_Cyrl_UZ: "Uzbek (Cyrillic, Uzbekistan)",
uz_Latn: "Uzbek (Latin)",
uz_Latn_UZ: "Uzbek (Latin, Uzbekistan)",
uz: "Uzbek",
vi_VN: "Vietnamese (Vietnam)",
vi: "Vietnamese",
vun_TZ: "Vunjo (Tanzania)",
vun: "Vunjo",
cy_GB: "Welsh (United Kingdom)",
cy: "Welsh",
yo_NG: "Yoruba (Nigeria)",
yo: "Yoruba",
zu_ZA: "Zulu (South Africa)",
zu: "Zulu"
}