tools: align locale diagnostics and generate native language names

tools/preview/parity_test.go

@@ -294,6 +294,7 @@ func TestSubjectSplitAndMissingFixture(t *testing.T) {
 
 func TestPreviewBundles(t *testing.T) {
 	result := &PreviewResult{Rendered: map[string]map[string]map[string]string{"en-US": {"test": {"activate": "<body>hello</body>"}}, "zh-CN": {"test": {"activate": "<body>你好</body>"}}}, Languages: []string{"en-US", "zh-CN"}, Themes: []string{"test"}, Registry: map[string]map[string]string{}, Upstream: "v28.0.0"}
+	result.LanguageNames = languageNames(result.Languages)
 	output := filepath.Join(t.TempDir(), "rendered.js")
 	if err := WriteRenderedJS(result, output); err != nil {
 		t.Fatal(err)
@@ -305,6 +306,9 @@ func TestPreviewBundles(t *testing.T) {
 	if bytes.Contains(manifest, []byte("<body>")) {
 		t.Fatal("manifest embeds mail payload")
 	}
+	if !bytes.Contains(manifest, []byte(`"language_names":`)) || !bytes.Contains(manifest, []byte(fmt.Sprintf("%q:%q", "en-US", result.LanguageNames["en-US"]))) {
+		t.Fatal("manifest does not include generated native language names")
+	}
 	for _, lang := range result.Languages {
 		data, err := os.ReadFile(filepath.Join(filepath.Dir(output), "rendered", lang+".js"))
 		if err != nil {