Add nan-TW locale

app/helpers/languages_helper.rb

@@ -209,6 +209,7 @@ module LanguagesHelper
   # but a language family in spite of sharing the main locale code
   # We need to be able to filter these
   ISO_639_1_REGIONAL = {
+    'nan-TW': ['Taiwanese (Hokkien)', '臺語（Hô-ló話）'].freeze,
     'zh-CN': ['Chinese (China)', '简体中文'].freeze,
     'zh-HK': ['Chinese (Hong Kong)', '繁體中文（香港）'].freeze,
     'zh-TW': ['Chinese (Taiwan)', '繁體中文（臺灣）'].freeze,

config/initializers/i18n.rb

@@ -62,6 +62,7 @@ Rails.application.configure do
     :mr,
     :ms,
     :my,
+    :'nan-TW',
     :nl,
     :nn,
     :no,