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/container.md

@@ -0,0 +1,40 @@
+---
+title: 容器（Container）
+id: container
+date: 2018-04-12
+full_link: /zh/docs/concepts/overview/what-is-kubernetes/#why-containers
+short_description: >
+  容器是可移植、可执行的轻量级的镜像，镜像中包含软件及其相关依赖。
+
+aka: 
+tags:
+- fundamental
+- workload
+---
+
+<!--
+title: Container
+id: container
+date: 2018-04-12
+full_link: /docs/concepts/containers/
+short_description: >
+  A lightweight and portable executable image that contains software and all of its dependencies.
+
+aka: 
+tags:
+- fundamental
+- workload
+-->
+
+<!--
+ A lightweight and portable executable image that contains software and all of its dependencies.
+-->
+容器是可移植、可执行的轻量级的镜像，包含其中的软件及其相关依赖。
+
+<!--more--> 
+
+<!--
+Containers decouple applications from underlying host infrastructure to make deployment easier in different cloud or OS environments, and for easier scaling.
+-->
+容器使应用和底层的主机基础设施解耦，降低了应用在不同云环境或者操作系统上的部署难度，便于应用扩展。
+