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/kubectl.md

@@ -0,0 +1,47 @@
+---
+title: Kubectl
+id: kubectl
+date: 2018-04-12
+full_link: /docs/user-guide/kubectl-overview/
+short_description: >
+  kubectl 是用来和 Kubernetes 集群进行通信的命令行工具。
+
+aka: 
+- kubectl
+tags:
+- tool
+- fundamental
+---
+
+<!--
+---
+title: Kubectl
+id: kubectl
+date: 2018-04-12
+full_link: /docs/user-guide/kubectl-overview/
+short_description: >
+  A command line tool for communicating with a Kubernetes cluster.
+
+aka: 
+- kubectl
+tags:
+- tool
+- fundamental
+---
+-->
+
+<!--
+Command line tool for communicating with a Kubernetes cluster's
+{{< glossary_tooltip text="control plane" term_id="control-plane" >}},
+using the Kubernetes API.
+-->
+
+kubectl 是使用 Kubernetes API 与 Kubernetes 
+集群的{{<glossary_tooltip text="控制面" term_id="control-plane" >}}进行通信的命令行工具。
+<!--more--> 
+
+<!--
+You can use `kubectl` to create, inspect, update, and delete Kubernetes objects.
+-->
+
+你可以使用 `kubectl` 创建、检视、更新和删除 Kubernetes 对象。