fix(web): prevent duplicate i18n languageChanged listeners (#13218)

### What problem does this PR solve?

As title.

### Type of change

- [x] Refactoring

web/src/app.tsx

@@ -69,10 +69,18 @@ function Root({ children }: React.PropsWithChildren) {
     }
   }, []);
 
-  i18n.on('languageChanged', function (lng: string) {
-    storage.setLanguage(lng);
-    document.documentElement.lang = lng;
-  });
+  useEffect(() => {
+    const handleLanguageChanged = (lng: string) => {
+      storage.setLanguage(lng);
+      document.documentElement.lang = lng;
+    };
+
+    i18n.on('languageChanged', handleLanguageChanged);
+
+    return () => {
+      i18n.off('languageChanged', handleLanguageChanged);
+    };
+  }, []);
 
   return (
     <SidebarProvider className="h-full">