Add RemoteTranslation validations

- Validate that locale is a valid locale for RemoteTranslation Client.
- RemoteTranslation can only be created for resources that do not have the requested
language translated

spec/shared/features/remotely_translatable.rb

@@ -10,7 +10,7 @@ shared_examples "remotely_translatable" do |factory_name, path_name, path_argume
   before do
     Setting["feature.remote_translations"] = true
     available_locales_response = %w[de en es fr pt zh-Hans]
-    expect(RemoteTranslations::Microsoft::AvailableLocales).to receive(:available_locales).at_most(3).times.
+    expect(RemoteTranslations::Microsoft::AvailableLocales).to receive(:available_locales).at_most(4).times.
                                                             and_return(available_locales_response)
     allow(Rails.application.secrets).to receive(:microsoft_api_key).and_return("123")
   end