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/admission-controller.md

@@ -0,0 +1,50 @@
+---
+title: 准入控制器（Admission Controller）
+id: admission-controller
+date: 2019-06-28
+full_link: /zh/docs/reference/access-authn-authz/admission-controllers/
+short_description: >
+ 在对象持久化之前拦截 Kubernetes Api 服务器请求的一段代码
+aka:
+tags:
+- extension
+- security
+---
+
+
+<!--
+---
+title: Admission Controller
+id: admission-controller
+date: 2019-06-28
+full_link: /docs/reference/access-authn-authz/admission-controllers/
+short_description: >
+  A piece of code that intercepts requests to the Kubernetes API server prior to persistence of the object.
+
+aka:
+tags:
+- extension
+- security
+---																										 
+-->
+
+
+<!--
+A piece of code that intercepts requests to the Kubernetes API server prior to persistence of the object.
+-->
+在对象持久化之前拦截 Kubernetes Api 服务器请求的一段代码
+
+<!--more-->
+
+<!--
+Admission controllers are configurable for the Kubernetes API server and may be “validating”, “mutating”, or
+both. Any admission controller may reject the request. Mutating controllers may modify the objects they admit;
+validating controllers may not.
+
+* [Admission controllers in the Kubernetes documentation](/docs/reference/access-authn-authz/admission-controllers/)
+-->
+
+准入控制器可针对 Kubernetes Api 服务器进行配置，可以执行验证，变更或两者都执行。任何准入控制器都可以拒绝访问请求。
+变更（mutating）控制器可以修改其允许的对象，验证（validating）控制器则不可以。
+
+* [Kubernetes 文档中的准入控制器](/zh/docs/reference/access-authn-authz/admission-controllers/)