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/pod.md

@@ -0,0 +1,43 @@
+---
+title: Pod
+id: pod
+date: 2018-04-12
+full_link: /docs/concepts/workloads/pods/pod-overview/
+short_description: >
+  Pod 表示你的集群上一组正在运行的容器。
+
+aka: 
+tags:
+- core-object
+- fundamental
+---
+
+<!--
+---
+title: Pod
+id: pod
+date: 2018-04-12
+full_link: /docs/concepts/workloads/pods/pod-overview/
+short_description: >
+  A Pod represents a set of running containers in your cluster.
+
+aka: 
+tags:
+- core-object
+- fundamental
+---
+-->
+
+<!--
+ The smallest and simplest Kubernetes object. A Pod represents a set of running {{< glossary_tooltip text="containers" term_id="container" >}} on your cluster.
+-->
+
+ Pod 是 Kubernetes 的原子对象。Pod 表示您的集群上一组正在运行的{{< glossary_tooltip text="容器（containers）" term_id="container" >}}。
+
+<!--more--> 
+
+<!--
+A Pod is typically set up to run a single primary container. It can also run optional sidecar containers that add supplementary features like logging. Pods are commonly managed by a {{< glossary_tooltip term_id="deployment" >}}.
+-->
+
+通常创建 Pod 是为了运行单个主容器。Pod 还可以运行可选的边车（sidecar）容器，以添加诸如日志记录之类的补充特性。通常用 {{< glossary_tooltip term_id="deployment" >}} 来管理 Pod。