refactor(i18n): use lowercase language codes

Related with this Hugo issue: https://github.com/gohugoio/hugo/issues/7344

exampleSite/config.yaml

@@ -9,7 +9,7 @@ languages:
         languageName: English
         title: Example Site
         weight: 1
-    zh-CN:
+    zh-cn:
         languageName: 中文
         title: 演示站点
         weight: 2
@@ -26,7 +26,7 @@ disqusShortname: hugo-theme-stack
 googleAnalytics:
 
 # Theme i18n support
-# Available values: en, fr, id, ja, ko, pt-br, zh-CN, zh-TW, es, de, nl, it, th, el, uk, ar
+# Available values: en, fr, id, ja, ko, pt-br, zh-cn, zh-tw, es, de, nl, it, th, el, uk, ar
 DefaultContentLanguage: en
 
 # Set hasCJKLanguage to true if DefaultContentLanguage is in [zh-cn ja ko]