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/certificate.md

@@ -0,0 +1,43 @@
+---
+title: 证书（Certificate）
+id: certificate
+date: 2018-04-12
+full_link: /zh/docs/tasks/tls/managing-tls-in-a-cluster/
+short_description: >
+  证书是个安全加密文件，用来确认对 Kubernetes 集群访问的合法性。
+
+aka: 
+tags:
+- security
+---
+
+<!--
+---
+title: Certificate
+id: certificate
+date: 2018-04-12
+full_link: /docs/tasks/tls/managing-tls-in-a-cluster/
+short_description: >
+  A cryptographically secure file used to validate access to the Kubernetes cluster.
+
+aka: 
+tags:
+- security
+---
+-->
+
+
+<!--
+ A cryptographically secure file used to validate access to the Kubernetes cluster.
+-->
+
+证书是个安全加密文件，用来确认对 Kubernetes 集群访问的合法性。
+
+<!--more--> 
+
+<!--
+Certificates enable applications within a Kubernetes cluster to access the Kubernetes API securely. Certificates validate that clients are allowed to access the API.
+-->
+
+证书可以让 Kubernetes 集群中运行的应用程序安全的访问 Kubernetes API。证书可以确认客户端是否被允许访问 API。
+