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/extensions.md

@@ -0,0 +1,41 @@
+---
+title: 扩展组件（Extensions）
+id: Extensions
+date: 2019-02-01
+full_link: /zh/docs/concepts/extend-kubernetes/extend-cluster/#extensions
+short_description: >
+  扩展组件是扩展并与 Kubernetes 深度集成以支持新型硬件的软件组件。
+aka:
+tags:
+- fundamental
+- extension
+---
+
+<!--
+title: Extensions
+id: Extensions
+date: 2019-02-01
+full_link: /docs/concepts/extend-kubernetes/extend-cluster/#extensions
+short_description: >
+  Extensions are software components that extend and deeply integrate with Kubernetes to support new types of hardware.
+
+aka:
+tags:
+- fundamental
+- extension
+-->
+<!--
+ Extensions are software components that extend and deeply integrate with Kubernetes to support new types of hardware.
+-->
+  扩展组件是扩展并与 Kubernetes 深度集成以支持新型硬件的软件组件。
+
+<!--more-->
+
+<!--
+Many cluster administrators use a hosted or distribution instance of Kubernetes. These clusters come with extensions pre-installed. As a result, most Kubernetes users will not need to install [extensions](/docs/concepts/extend-kubernetes/extend-cluster/#extensions) and even fewer users will need to author new ones.
+-->
+
+许多集群管理员会使用托管的 Kubernetes 或其某种发行包，这些集群预装了扩展。
+因此，大多数 Kubernetes 用户将不需要
+安装[扩展组件](/zh/docs/concepts/extend-kubernetes/extend-cluster/#extensions)，
+需要编写新的扩展组件的用户就更少了。