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/platform-developer.md

@@ -0,0 +1,50 @@
+---
+title: 平台开发人员（Platform Developer）
+id: platform-developer
+date: 2018-04-12
+full_link: 
+short_description: >
+  定制 Kubernetes 平台以满足自己的项目需求的人。
+
+aka: 
+tags:
+- user-type
+---
+
+<!--
+title: Platform Developer
+id: platform-developer
+date: 2018-04-12
+full_link: 
+short_description: >
+  A person who customizes the Kubernetes platform to fit the needs of their project.
+
+aka: 
+tags:
+- user-type
+-->
+
+<!--
+A person who customizes the Kubernetes platform to fit the needs of their project.
+-->
+
+定制 Kubernetes 平台以满足自己的项目需求的人。
+
+<!--more--> 
+
+<!--
+A platform developer may, for example, use [Custom Resources](/docs/concepts/extend-kubernetes/api-extension/custom-resources/) or
+[Extend the Kubernetes API with the aggregation layer](/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/)
+to add functionality to their instance of Kubernetes, specifically for their application.
+Some Platform Developers are also {{< glossary_tooltip text="contributors" term_id="contributor" >}} and
+develop extensions which are contributed to the Kubernetes community.
+Others develop closed-source commercial or site-specific extensions.
+-->
+
+平台开发人员可以使用[定制资源](/zh/docs/concepts/extend-kubernetes/api-extension/custom-resources/)
+或[使用汇聚层扩展 Kubernetes API](/zh/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/)
+来为其 Kubernetes 实例增加功能，特别是为其应用程序添加功能。
+一些平台开发人员也是 kubernetes {{< glossary_tooltip text="贡献者" term_id="contributor" >}}，
+他们会开发贡献给 Kubernetes 社区的扩展。
+另一些平台开发人员则开发封闭源代码的商业扩展或用于特定网站的扩展。
+