according to https://git.pleroma.social/pleroma/pleroma-fe/-/merge_requests/1841#note_101513 by tusooa

src/services/locale/locale.service.js

@@ -3,7 +3,6 @@ import ISO6391 from 'iso-639-1'
 import _ from 'lodash'
 
 const specialLanguageCodes = {
-  nan: 'nan-TW',
   ja_easy: 'ja',
   zh_Hant: 'zh-HANT',
   zh: 'zh-Hans'
@@ -20,7 +19,7 @@ const internalToBackendLocaleMulti = codes => {
 const getLanguageName = (code) => {
   const specialLanguageNames = {
     ja_easy: 'やさしいにほんご',
-    nan: '臺語（閩南語）',
+    nan_TW: '臺語（閩南語）',
     zh: '简体中文',
     zh_Hant: '繁體中文'
   }