Convert /instances/* controller specs to request specs (#27988)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

spec/requests/api/v1/instances/translation_languages_spec.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'Translation Languages' do
+  describe 'GET /api/v1/instances/translation_languages' do
+    context 'when no translation service is configured' do
+      it 'returns empty language matrix', :aggregate_failures do
+        get api_v1_instance_translation_languages_path
+
+        expect(response)
+          .to have_http_status(200)
+
+        expect(body_as_json)
+          .to eq({})
+      end
+    end
+
+    context 'when a translation service is configured' do
+      before { configure_translation_service }
+
+      it 'returns language matrix', :aggregate_failures do
+        get api_v1_instance_translation_languages_path
+
+        expect(response)
+          .to have_http_status(200)
+
+        expect(body_as_json)
+          .to eq({ und: %w(en de), en: ['de'] })
+      end
+
+      private
+
+      def configure_translation_service
+        allow(TranslationService).to receive_messages(configured?: true, configured: service_double)
+      end
+
+      def service_double
+        instance_double(TranslationService::DeepL, languages: { nil => %w(en de), 'en' => ['de'] })
+      end
+    end
+  end
+end