make users explicitly define localized setting values so it's clear which locale is mocked

spec/support/shared/with_settings.rb

@@ -53,6 +53,12 @@ RSpec.configure do |config|
 
         raise "#{k} is not a valid setting" unless Setting.respond_to?(name)
 
+        expect(name).not_to start_with("localized_"), ->() do
+          base = name[10..-1]
+
+          "Don't use `#{name}` in `with_settings`. Do this: `with_settings: { #{base}: { \"en\" => \"#{v}\" } }`"
+        end
+
         allow(Setting).to receive(:[]).with(name).and_return v
         allow(Setting).to receive(:[]).with(name.to_sym).and_return v
       end

spec_legacy/functional/user_mailer_spec.rb

@@ -396,7 +396,9 @@ describe UserMailer, type: :mailer do
   context 'layout',
           with_settings: {
             available_languages: [:en, :de],
-            localized_emails_header: 'deutscher header'
+            emails_header: {
+              "de" => 'deutscher header'
+            }
           } do
     it 'should include the emails_header depeding on the locale' do
       user = FactoryBot.create(:user, language: :de)