diff --git a/packages/i18n/src/client.test.ts b/packages/i18n/src/client.test.ts
new file mode 100644
index 0000000..c7b0d28
--- /dev/null
+++ b/packages/i18n/src/client.test.ts
@@ -0,0 +1,16 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { describe, expect, it } from "vitest";
+import { i18n, initI18nClient } from "./index.ts";
+
+describe("initI18nClient", () => {
+  it("normalizes regional html lang tags to supported languages", () => {
+    document.documentElement.lang = "de-DE";
+
+    initI18nClient();
+
+    expect(i18n.language).toBe("de");
+    expect(i18n.t("journal:nav.login")).toBe("Anmelden");
+  });
+});
diff --git a/packages/i18n/src/index.ts b/packages/i18n/src/index.ts
index 5db3adb..7634071 100644
--- a/packages/i18n/src/index.ts
+++ b/packages/i18n/src/index.ts
@@ -64,6 +64,7 @@ export function initI18nClient() {
       ...commonOptions,
       detection: {
         order: ["htmlTag", "navigator"],
+        convertDetectedLanguage: (lng: string) => matchSupportedLng(lng),
         caches: [],
       },
     });