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/secret.md

@@ -0,0 +1,47 @@
+---
+title: Secret
+id: secret
+date: 2018-04-12
+full_link: /zh/docs/concepts/configuration/secret/
+short_description: >
+  Secret 用于存储敏感信息，如密码、 OAuth 令牌和 SSH 密钥。
+
+aka: 
+tags:
+- core-object
+- security
+---
+
+<!--
+---
+title: Secret
+id: secret
+date: 2018-04-12
+full_link: /docs/concepts/configuration/secret/
+short_description: >
+  Stores sensitive information, such as passwords, OAuth tokens, and ssh keys.
+
+aka: 
+tags:
+- core-object
+- security
+---
+-->
+
+<!--
+ Stores sensitive information, such as passwords, OAuth tokens, and ssh keys.
+-->
+
+ Secret 用于存储敏感信息，如密码、 OAuth 令牌和 SSH 密钥。
+
+<!--more--> 
+
+<!--
+Allows for more control over how sensitive information is used and reduces the risk of accidental exposure. Secret values are encoded as base64 strings and stored unencrypted by default, but can be configured to be [encrypted at rest](/docs/tasks/administer-cluster/encrypt-data/#ensure-all-secrets-are-encrypted). A {{< glossary_tooltip text="Pod" term_id="pod" >}} references the secret as a file in a volume mount or by the kubelet pulling images for a pod. Secrets are great for confidential data and [ConfigMaps](/docs/tasks/configure-pod-container/configure-pod-configmap/) for non-confidential data.
+-->
+
+Secret 允许用户对如何使用敏感信息进行更多的控制，并减少信息意外暴露的风险。
+默认情况下，Secret 值被编码为 base64 字符串并以非加密的形式存储，但可以配置为
+[静态加密（Encrypt at rest）](/zh/docs/tasks/administer-cluster/encrypt-data/#ensure-all-secrets-are-encrypted)。
+{{< glossary_tooltip text="Pod" term_id="pod" >}} 通过挂载卷中的文件的方式引用 Secret，或者通过 kubelet 为 pod 拉取镜像时引用。
+Secret 非常适合机密数据使用，而 [ConfigMaps](/zh/docs/tasks/configure-pod-container/configure-pod-configmap/) 适用于非机密数据。