perf(i18n): set the global default locales to "en" (#979)

Avoid missing labels caused by languages ​​outside of locales.

assets/404.html

@@ -9,6 +9,8 @@ redirect_from:
   - /posts/
 ---
 
+{% include lang.html %}
+
 <div class="lead">
-  <p>{{site.data.locales[site.lang].not_found.statment }}</p>
+  <p>{{ site.data.locales[lang].not_found.statment }}</p>
 </div>