Fix language filter codes (#4841)

* Fix language filter codes

CLD3 returns BCP-47 language identifier, filter settings expect
identifiers in the ISO 639-1 format. Convert between formats,
and exclude duplicate languages from filter choices (zh-CN->zh)

* Fix zh name

app/helpers/settings_helper.rb

@@ -30,6 +30,7 @@ module SettingsHelper
     th: 'ภาษาไทย',
     tr: 'Türkçe',
     uk: 'Українська',
+    zh: '中文',
     'zh-CN': '简体中文',
     'zh-HK': '繁體中文（香港）',
     'zh-TW': '繁體中文（臺灣）',
@@ -39,6 +40,10 @@ module SettingsHelper
     HUMAN_LOCALES[locale]
   end
 
+  def filterable_languages
+    I18n.available_locales.map { |locale| locale.to_s.split('-').first.to_sym }.uniq
+  end
+
   def hash_to_object(hash)
     HashObject.new(hash)
   end