Replace Status#translatable? with language matrix in separate endpoint (#24037)

spec/controllers/api/v1/instances/translation_languages_controller_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Api::V1::Instances::TranslationLanguagesController do
+  describe 'GET #show' do
+    context 'when no translation service is configured' do
+      it 'returns empty language matrix' do
+        get :show
+
+        expect(response).to have_http_status(200)
+        expect(body_as_json).to eq({})
+      end
+    end
+
+    context 'when a translation service is configured' do
+      before do
+        service = instance_double(TranslationService::DeepL, languages: { nil => %w(en de), 'en' => ['de'] })
+        allow(TranslationService).to receive(:configured?).and_return(true)
+        allow(TranslationService).to receive(:configured).and_return(service)
+      end
+
+      it 'returns language matrix' do
+        get :show
+
+        expect(response).to have_http_status(200)
+        expect(body_as_json).to eq({ und: %w(en de), en: ['de'] })
+      end
+    end
+  end
+end