Add Vietnamese option to language dropdown (#1244)

Author: phainv

themes/vue/layout/partials/language_dropdown.ejs

@@ -7,5 +7,6 @@
     <li><a href="https://kr.vuejs.org/<%- page.path %>" class="nav-link" target="_blank">한국어</a></li>
     <li><a href="https://br.vuejs.org/<%- page.path %>" class="nav-link" target="_blank">Português</a></li>
     <li><a href="https://fr.vuejs.org/<%- page.path %>" class="nav-link" target="_blank">Français</a></li>
+    <li><a href="https://vi.vuejs.org/<%- page.path %>" class="nav-link" target="_blank">Tiếng Việt</a></li>
   </ul>
 </li>