Make HTML lang attribute dynamic based on selected language

- Create HtmlLang client component that updates document.documentElement.lang
- Add HtmlLang component to layout.tsx to sync HTML lang attribute with locale
- HTML lang now updates automatically when user changes language

frontend/app/components/HtmlLang.tsx

@@ -0,0 +1,19 @@
+"use client";
+
+import { useEffect } from "react";
+import { useLanguage } from "../hooks/useLanguage";
+
+/**
+ * Client component that updates the HTML lang attribute based on the selected language.
+ * This must be a client component because it uses hooks and DOM manipulation.
+ */
+export function HtmlLang() {
+  const { locale } = useLanguage();
+
+  useEffect(() => {
+    // Update the HTML lang attribute when locale changes
+    document.documentElement.lang = locale;
+  }, [locale]);
+
+  return null;
+}