fix nan-tw not listed in SUPPORTED_LOCALES (#37721)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/helpers/languages_helper.rb

@@ -223,7 +223,14 @@ module LanguagesHelper
     'zh-YUE': ['Cantonese', '廣東話'].freeze,
   }.freeze
 
-  SUPPORTED_LOCALES = {}.merge(ISO_639_1).merge(ISO_639_1_REGIONAL).merge(ISO_639_3).freeze
+  # Since nan is not translated but nan-TW is translated,
+  # to enable the ISO-639-3 language-code with the regional variant but no
+  # official name, we use a specific hash for nan-TW
+  ISO_639_3_REGIONAL = {
+    'nan-TW': ['Hokkien (Taiwan)', '臺語 (Hô-ló話)'].freeze,
+  }.freeze
+
+  SUPPORTED_LOCALES = {}.merge(ISO_639_1).merge(ISO_639_1_REGIONAL).merge(ISO_639_3).merge(ISO_639_3_REGIONAL).freeze
 
   # For ISO-639-1 and ISO-639-3 language codes, we have their official
   # names, but for some translations, we need the names of the
@@ -233,7 +240,6 @@ module LanguagesHelper
     'es-AR': 'Español (Argentina)',
     'es-MX': 'Español (México)',
     'fr-CA': 'Français (Canadien)',
-    'nan-TW': '臺語 (Hô-ló話)',
     'pt-BR': 'Português (Brasil)',
     'pt-PT': 'Português (Portugal)',
     'sr-Latn': 'Srpski (latinica)',