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/kubernetes-api.md

@@ -0,0 +1,47 @@
+---
+title: Kubernetes API
+id: kubernetes-api
+date: 2018-04-12
+full_link: /zh/docs/concepts/overview/kubernetes-api/
+short_description: >
+  Kubernetes API 是通过 RESTful 接口提供 Kubernetes 功能服务并负责集群状态存储的应用程序。
+
+aka: 
+tags:
+- fundamental
+- architecture
+---
+
+<!--
+---
+title: Kubernetes API
+id: kubernetes-api
+date: 2018-04-12
+full_link: /docs/concepts/overview/kubernetes-api/
+short_description: >
+  The application that serves Kubernetes functionality through a RESTful interface and stores the state of the cluster.
+
+aka: 
+tags:
+- fundamental
+- architecture
+---
+-->
+
+<!--
+ The application that serves Kubernetes functionality through a RESTful interface and stores the state of the cluster.
+-->
+
+Kubernetes API 是通过 RESTful 接口提供 Kubernetes 功能服务并负责集群状态存储的应用程序。
+
+<!--more--> 
+
+<!--
+Kubernetes resources and "records of intent" are all stored as API objects, and modified via RESTful calls to the API. The API allows configuration to be managed in a declarative way. Users can interact with the Kubernetes API directly, or via tools like `kubectl`. The core Kubernetes API is flexible and can also be extended to support custom resources.
+-->
+
+Kubernetes 资源和"意向记录"都是作为 API 对象储存的，并可以通过调用 RESTful 风格的 API 进行修改。
+API 允许以声明方式管理配置。
+用户可以直接和 Kubernetes API 交互，也可以通过 `kubectl` 这样的工具进行交互。
+核心的 Kubernetes API 是很灵活的，可以扩展以支持定制资源。
+