diff --git a/modules/core/localizer.js b/modules/core/localizer.js
index ebb3ecb22..f21a3ad37 100644
--- a/modules/core/localizer.js
+++ b/modules/core/localizer.js
@@ -354,7 +354,7 @@ export function coreLocalizer() {
     };
 
     localizer.htmlForLocalizedText = function(text, localeCode) {
-        return `<span class="localized-text" lang="${localeCode || 'unknown'}">${text}</span>`;
+        return `<span class="localized-text" lang="${localeCode || 'und'}">${text}</span>`;
     };
 
     localizer.languageName = (code, options) => {