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/kube-apiserver.md

@@ -0,0 +1,47 @@
+---
+title: kube-apiserver
+id: kube-apiserver
+date: 2018-04-12
+full_link: /zh/docs/reference/command-line-tools-reference/kube-apiserver/
+short_description: >
+  提供 Kubernetes API 服务的控制面组件。
+
+aka: 
+tags:
+- architecture
+- fundamental
+---
+
+<!--
+title: kube-apiserver
+id: kube-apiserver
+date: 2018-04-12
+full_link: /zh/docs/reference/command-line-tools-reference/kube-apiserver/
+short_description: >
+  Control plane component that serves the Kubernetes API. 
+
+aka: 
+tags:
+- architecture
+- fundamental
+-->
+
+<!--
+ The API server is a component of the Kubernetes
+{{< glossary_tooltip text="control plane" term_id="control-plane" >}} that exposes the Kubernetes API.
+The API server is the front end for the Kubernetes control plane.
+-->
+API 服务器是 Kubernetes {{< glossary_tooltip text="控制平面" term_id="control-plane" >}}的组件，
+该组件负责公开了 Kubernetes API，负责处理接受请求的工作。
+API 服务器是 Kubernetes 控制平面的前端。
+
+<!--more--> 
+
+<!--
+The main implementation of a Kubernetes API server is [kube-apiserver](/docs/reference/generated/kube-apiserver/).
+kube-apiserver is designed to scale horizontally&mdash;that is, it scales by deploying more instances.
+You can run several instances of kube-apiserver and balance traffic between those instances.
+-->
+Kubernetes API 服务器的主要实现是 [kube-apiserver](/zh/docs/reference/command-line-tools-reference/kube-apiserver/)。
+`kube-apiserver` 设计上考虑了水平扩缩，也就是说，它可通过部署多个实例来进行扩缩。
+你可以运行 `kube-apiserver` 的多个实例，并在这些实例之间平衡流量。