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/storage-class.md

@@ -0,0 +1,46 @@
+---
+title: StorageClass
+id: storageclass
+date: 2018-04-12
+full_link: /zh/docs/concepts/storage/storage-classes/
+short_description: >
+  StorageClass 是管理员用来描述可用的不同存储类型的一种方法。
+
+aka: 
+tags:
+- core-object
+- storage
+---
+
+
+<!--
+---
+title: Storage Class
+id: storageclass
+date: 2018-04-12
+full_link: /docs/concepts/storage/storage-classes
+short_description: >
+  A StorageClass provides a way for administrators to describe different available storage types.
+
+aka:
+tags:
+- core-object
+- storage
+---
+-->
+
+<!--
+ A StorageClass provides a way for administrators to describe different available storage types.
+-->
+ StorageClass 是管理员用来描述不同的可用存储类型的一种方法。
+
+<!--more--> 
+
+<!--
+StorageClasses can map to quality-of-service levels, backup policies, or to arbitrary policies determined by cluster administrators. Each StorageClass contains the fields `provisioner`, `parameters`, and `reclaimPolicy`, which are used when a {{< glossary_tooltip text="Persistent Volume" term_id="persistent-volume" >}} belonging to the class needs to be dynamically provisioned. Users can request a particular class using the name of a StorageClass object.
+-->
+
+StorageClass 可以映射到服务质量等级（QoS）、备份策略、或者管理员任意定义的策略。
+每个 StorageClass 对象包含的字段有 `provisioner`、`parameters` 和 `reclaimPolicy`。
+动态制备该存储类别的{{< glossary_tooltip text="持久卷" term_id="persistent-volume" >}}时需要用到这些字段值。
+通过设置 StorageClass 对象的名称，用户可以请求特定存储类别。