FEATURE: Add English (UK) as locale (#11768)

* "English" gets renamed into "English (US)"
* "English (UK)" replaces "English"

@discourse-translator-bot keep_translations_and_approvals

lib/js_locale_helper.rb

@@ -195,8 +195,7 @@ module JsLocaleHelper
   end
 
   MOMENT_LOCALE_MAPPING ||= {
-    "hy" => "hy-am",
-    "en" => "en-gb"
+    "hy" => "hy-am"
   }
 
   def self.find_moment_locale(locale_chain, timezone_names: false)
@@ -213,7 +212,6 @@ module JsLocaleHelper
       # moment.js uses a different naming scheme for locale files
       locale.tr('_', '-').downcase
     end
-
   end
 
   def self.find_message_format_locale(locale_chain, fallback_to_english:)

lib/site_settings/defaults_provider.rb

@@ -4,7 +4,7 @@ module SiteSettings; end
 
 # A cache for providing default value based on site locale
 class SiteSettings::DefaultsProvider
-  DEFAULT_LOCALE = 'en_US'
+  DEFAULT_LOCALE = 'en'
 
   def initialize(site_setting)
     @site_setting = site_setting