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/namespace.md

@@ -0,0 +1,45 @@
+---
+title: 名字空间（Namespace）
+id: namespace
+date: 2018-04-12
+full_link: /zh/docs/concepts/overview/working-with-objects/namespaces/
+short_description: >
+  名字空间是 Kubernetes 用来支持隔离单个集群中的资源组的一种抽象。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+---
+title: Namespace
+id: namespace
+date: 2018-04-12
+full_link: /docs/concepts/overview/working-with-objects/namespaces/
+short_description: >
+  An abstraction used by Kubernetes to support isolation of groups of resources within a single cluster.
+
+aka: 
+tags:
+- fundamental
+---
+-->
+
+<!--
+ An abstraction used by Kubernetes to support isolation of groups of resources within a single {{< glossary_tooltip text="cluster" term_id="cluster" >}}.
+-->
+
+名字空间是 Kubernetes 用来支持隔离单个 {{< glossary_tooltip text="集群" term_id="cluster" >}}中的资源组的一种抽象。
+
+<!--more--> 
+
+<!--
+Namespaces are used to organize objects in a cluster and provide a way to divide cluster resources. Names of resources need to be unique within a namespace, but not across namespaces. Namespace-based scoping is applicable only for namespaced objects _(e.g. Deployments, Services, etc)_ and not for cluster-wide objects _(e.g. StorageClass, Nodes, PersistentVolumes, etc)_.
+-->
+
+名字空间用来组织集群中对象，并为集群资源划分提供了一种方法。
+同一名字空间内的资源名称必须唯一，但跨名字空间时不作要求。
+基于名字空间的作用域限定仅适用于名字空间作用域的对象（例如 Deployment、Services 等），
+而不适用于集群作用域的对象（例如 StorageClass、Node、PersistentVolume 等）。
+在一些文档里名字空间也称为命名空间。