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/persistent-volume.md

@@ -0,0 +1,48 @@
+---
+title: 持久卷（Persistent Volume）
+id: persistent-volume
+date: 2018-04-12
+full_link: /zh/docs/concepts/storage/persistent-volumes/
+short_description: >
+  持久卷是代表集群中一块存储空间的 API 对象。 它是通用的、可插拔的、并且不受单个 Pod 生命周期约束的持久化资源。
+
+aka: 
+tags:
+- core-object
+- storage
+---
+
+<!--
+---
+title: Persistent Volume
+id: persistent-volume
+date: 2018-04-12
+full_link: /docs/concepts/storage/persistent-volumes/
+short_description: >
+  An API object that represents a piece of storage in the cluster. Available as a general, pluggable resource that persists beyond the lifecycle of any individual Pod.
+
+aka: 
+tags:
+- core-object
+- storage
+---
+-->
+
+<!--
+ An API object that represents a piece of storage in the cluster. Available as a general, pluggable resource that persists beyond the lifecycle of any individual {{< glossary_tooltip text="Pod" term_id="pod" >}}.
+-->
+
+持久卷是代表集群中一块存储空间的 API 对象。 它是通用的、可插拔的、并且不受单个 {{< glossary_tooltip text="Pod" term_id="pod" >}} 生命周期约束的持久化资源。
+
+<!--more--> 
+
+<!--
+PersistentVolumes (PVs) provide an API that abstracts details of how storage is provided from how it is consumed.
+PVs are used directly in scenarios where storage can be created ahead of time (static provisioning).
+For scenarios that require on-demand storage (dynamic provisioning), PersistentVolumeClaims (PVCs) are used instead.
+-->
+
+持久卷（PersistentVolumes，PV）提供了一个 API，该 API 对存储的供应方式细节进行抽象，令其与使用方式相分离。
+在提前创建存储（静态供应）的场景中，PV 可以直接使用。
+在按需提供存储（动态供应）的场景中，需要使用 PersistentVolumeClaims (PVCs)。
+