Here are the values of YOURLS_LANG people use in their YOURLS config.php (real life data fetch on April 2022)
| Locale |
|---|
| af_ZA |
| ar |
| ar-SA |
| ar_AR |
| ar_EG |
| ar_JO |
| ar_SA |
| ast_ES |
| at-DE |
| bg |
| bg_BG |
| BR |
| BR-pt |
| ca |
| ca_ES |
| ch-CN |
| chinese |
| ch_Zh |
| cn |
| cs |
| cs-CZ |
| cs_CS |
| cs_CZ |
| CZ |
| da-DK |
| da_DK |
| de |
| de-DE |
| deDE |
| de_AC |
| de_CH |
| de_DE |
| DK |
| dutch |
| el |
| el_GR |
| EN |
| en-CA |
| EN-en |
| en-GB |
| en-IN |
| en-us |
| eng |
| Englis |
| english |
| Enlish |
| en_AU |
| EN_en |
| en_GB |
| en_IN |
| en_UK |
| en_US |
| ES |
| es-ES |
| est |
| es_AR |
| es_CL |
| es_CO |
| es_ES |
| es_LA |
| es_MX |
| et |
| et_EE |
| euc-kr |
| e_DE |
| fa |
| fa-IR |
| false |
| fa_FA |
| fa_IR |
| fi_Fi |
| fr |
| fr-FR |
| France |
| French |
| fr_FR |
| gangsta_CA |
| GE |
| ger |
| german |
| Germany |
| harmonia |
| he |
| Hello |
| he_IL |
| hi |
| hi-IN |
| HR |
| HU |
| Hungarian |
| hu_HU |
| ID |
| id-ID |
| id_ID |
| ies |
| indonesia |
| indonesian |
| IR-fa |
| it |
| it-IT |
| italian |
| it_IT |
| ja |
| ja-jp |
| Japanese |
| ja_JP |
| ja_JP_test |
| JP |
| JP-ja |
| jp_JP |
| ko |
| ko-KR |
| korean |
| ko_KO |
| ko_KR |
| kr |
| kr_KR |
| locale |
| localize |
| LT |
| LV |
| lv_LV |
| nb_NO |
| Nederlands |
| NL |
| nl-nl |
| nl_BE |
| nl_NL |
| nn_NO |
| null |
| persian |
| pl |
| PL-pl |
| pl_PL |
| polish |
| Portuguese |
| PT |
| pt-br |
| pt-PT |
| pt-_R |
| pt_BR |
| pt_moe |
| pt_PT |
| pt_VR |
| ro |
| romana |
| ro_RO |
| ru |
| ru-RU |
| rus |
| Russian |
| ru_RU |
| s |
| se_SV |
| sk |
| sk_SK |
| sl |
| sl_SI |
| sp |
| SPANISH |
| sr |
| sr_RS |
| SV |
| sv-SE |
| Svenska |
| sv_SE |
| TH |
| th_TH |
| tr |
| tr-TR |
| true |
| tr_TR |
| turkish |
| twents |
| ua |
| ua_UA |
| uk |
| Ukrainian |
| uk_UA |
| us |
| us-en |
| us-US |
| US_EN |
| us_US |
| uz |
| Vi |
| vi-vn |
| vi_VN |
| vn |
| yourls-zh_CN |
| YOURLS-zh_TW |
| yourls_ |
| ZH |
| zh-CN |
| zh-Hans-CN |
| zh-Hant |
| zh-TW |
| zh-ZN |
| zhTW |
| zh_CH |
| zh_CN |
| zh_Hans |
| zh_Hant |
| zh_HK |
| zh_TW |
| zn_CN |