handle extra defaults just in case

es/es.11tydata.js

@@ -1,8 +1,19 @@
 module.exports = {
     lang: 'es',
     permalink: function (data) {
+        let stem = data.page.filePathStem;
+
+        // handle slug_override
         if (data.slug_override) {
-        return `/${data.lang}/${this.slugify(data.slug_override)}/`;
+            return `/${data.lang}/${this.slugify(data.slug_override)}/`;
         }
+
+        // handle index pages
+        if (stem.endsWith("/index")) {
+            return `${stem}.html`; // e.g., /es/index.html
+        }
+
+        // default for other pages
+        return `${stem}/index.html`; // e.g., /es/about/index.html
     }
-};
+};