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

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

_includes/lang.html

@@ -0,0 +1,8 @@
+{% comment %}
+  Detect appearance language and return it through variable "lang"
+{% endcomment %}
+{% if site.data.locales[site.lang] %}
+  {% assign lang = site.lang %}
+{% else %}
+  {% assign lang = 'en' %}
+{% endif %}