diff --git a/packages/app-shell/src/app-shell.js b/packages/app-shell/src/app-shell.js
index 7487f2058684fd7e381014c0f25d55444088804c..04372c544ea3b87bfd9794f7a10cbb8a60afe3ce 100644
--- a/packages/app-shell/src/app-shell.js
+++ b/packages/app-shell/src/app-shell.js
@@ -271,6 +271,10 @@ export class AppShell extends ScopedElementsMixin(AdapterLitElement) {
      * @param {string} lang
      */
     updateLangIfChanged(lang) {
+        // in case the language is unknown, fall back to the default
+        if (!i18n.languages.includes(lang)) {
+            lang = this.lang;
+        }
         if (this.lang !== lang) {
             this.lang = lang;
             this.router.update();