Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/docs/reference/glossary/application-developer.md

@@ -0,0 +1,42 @@
+---
+title: 应用开发者（Application Developer）
+id: application-developer
+date: 2018-04-12
+full_link: 
+short_description: >
+  编写可以在 Kubernetes 集群上运行的应用的人。
+
+aka: 
+tags:
+- user-type
+---
+
+<!--
+---
+title: Application Developer
+id: application-developer
+date: 2018-04-12
+full_link: 
+short_description: >
+  A person who writes an application that runs in a Kubernetes cluster.
+
+aka: 
+tags:
+- user-type
+---
+-->
+
+
+<!--
+ A person who writes an application that runs in a Kubernetes cluster.
+-->
+
+编写可以在 Kubernetes 集群上运行的应用的人。
+
+<!--more--> 
+
+<!--
+An application developer focuses on one part of an application. The scale of their focus may vary significantly in size.
+-->
+
+应用开发者专注于应用的某一部分。他们工作范围的大小有明显的差异。