core&ui: language selection bug fix

packages/ui-default/pages/problem_detail.page.jsx

@@ -3,6 +3,7 @@ import $ from 'jquery';
 import yaml from 'js-yaml';
 import React from 'react';
 import { createRoot } from 'react-dom/client';
+import Notification from 'vj/components/notification';
 import { downloadProblemSet } from 'vj/components/zipDownloader';
 import { NamedPage } from 'vj/misc/Page';
 import delay from 'vj/utils/delay';