link to Russian translation (#637)

Author: gbezyuk

themes/vue/layout/partials/language_dropdown.ejs

@@ -3,5 +3,6 @@
   <ul class="nav-dropdown">
     <li><a href="https://cn.vuejs.org/" class="nav-link" target="_blank">中文</a></li>
     <li><a href="https://jp.vuejs.org/" class="nav-link" target="_blank">日本語</a></li>
+    <li><a href="https://ru.vuejs.org/" class="nav-link" target="_blank">Русский</a></li>
   </ul>
 </li>