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

spec/helpers/settings_helper_spec.rb

@@ -4,10 +4,10 @@ require 'rails_helper'
 
 describe SettingsHelper do
   describe 'the HUMAN_LOCALES constant' do
-    it 'has the same number of keys as I18n locales exist' do
+    it 'includes all I18n locales' do
       options = I18n.available_locales
 
-      expect(described_class::HUMAN_LOCALES.keys).to eq(options)
+      expect(described_class::HUMAN_LOCALES.keys).to include(*options)
     end
   end