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-claim.md

@@ -0,0 +1,39 @@
+---
+title: 持久卷申领（Persistent Volume Claim）
+id: persistent-volume-claim
+date: 2018-04-12
+full_link: /zh/docs/concepts/storage/persistent-volumes/
+short_description: >
+  声明在持久卷中定义的存储资源，以便可以将其挂载为容器中的卷。
+
+aka: 
+tags:
+- core-object
+- storage
+---
+
+<!--
+title: Persistent Volume Claim
+id: persistent-volume-claim
+date: 2018-04-12
+full_link: /docs/concepts/storage/persistent-volumes/
+short_description: >
+  Claims storage resources defined in a PersistentVolume so that it can be mounted as a volume in a container.
+
+aka: 
+tags:
+- core-object
+- storage
+-->
+
+<!--
+ Claims storage resources defined in a {{< glossary_tooltip text="PersistentVolume" term_id="persistent-volume" >}} so that it can be mounted as a volume in a {{< glossary_tooltip text="container" term_id="container" >}}.
+-->
+申领{{< glossary_tooltip text="持久卷（PersistentVolume）" term_id="persistent-volume" >}}中定义的存储资源，以便可以将其挂载为{{< glossary_tooltip text="容器（container）" term_id="container" >}}中的卷。
+
+<!--more--> 
+
+<!--
+Specifies the amount of storage, how the storage will be accessed (read-only, read-write and/or exclusive) and how it is reclaimed (retained, recycled or deleted). Details of the storage itself are described in the PersistentVolume object.
+-->
+指定存储的数量，如何访问存储（只读、读写或独占）以及如何回收存储（保留、回收或删除）。存储本身的详细信息在 PersistentVolume 对象中。