Do not fetch environment variables to determine default locale (#6618)

The default locale is now set by config.

app/serializers/rest/instance_serializer.rb

@@ -48,7 +48,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
   end
 
   def languages
-    [ENV.fetch('DEFAULT_LOCALE', I18n.default_locale)]
+    [I18n.default_locale]
   end
 
   private

lib/tasks/assets.rake

 # frozen_string_literal: true
 
 def render_static_page(action, dest:, **opts)
-  I18n.with_locale(ENV['DEFAULT_LOCALE'] || I18n.default_locale) do
-    html = ApplicationController.render(action, opts)
-    File.write(dest, html)
-  end
+  html = ApplicationController.render(action, opts)
+  File.write(dest, html)
 end
 
 namespace :assets do