ui: handle session multi-lang

packages/ui-default/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@hydrooj/ui-default",
-  "version": "4.25.0",
+  "version": "4.25.1",
   "author": "undefined <i@undefined.moe>",
   "license": "AGPL-3.0",
   "main": "hydro.js",

packages/ui-default/templates/problem_detail.html

@@ -82,7 +82,7 @@
             <p>{{ _('You cannot submit for this problem because the contest is ended. You can click "Open in Problem Set" to view this problem in normal mode.') }}</p>
           </blockquote>
         {% endif %}
-        {{ pdoc.content|content(handler.user.viewLang, pdoc.html)|safe }}
+        {{ pdoc.content|content(handler.user.viewLang or handler.session.viewLang, pdoc.html)|safe }}
         </div>
       </div></div>
     </div>