tools: align locale diagnostics and generate native language names

tools/preview/languages.go

@@ -0,0 +1,21 @@
+package preview
+
+import (
+	"golang.org/x/text/language"
+	"golang.org/x/text/language/display"
+)
+
+// languageNames derives native names from CLDR for the generated preview locales.
+func languageNames(locales []string) map[string]string {
+	names := make(map[string]string, len(locales))
+	for _, locale := range locales {
+		name := locale
+		if tag, err := language.Parse(locale); err == nil {
+			if native := display.Self.Name(tag); native != "" {
+				name = native
+			}
+		}
+		names[locale] = name
+	}
+	return names
+}