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/csi.md

@@ -0,0 +1,52 @@
+---
+title: 容器存储接口（Container Storage Interface，CSI）
+id: csi
+date: 2018-06-25
+full_link: /zh/docs/concepts/storage/volumes/#csi
+short_description: >
+    容器存储接口 （CSI）定义了存储系统暴露给容器的标准接口。
+
+
+aka: 
+tags:
+- storage 
+---
+
+<!--
+---
+title: Container Storage Interface (CSI)
+id: csi
+date: 2018-06-25
+full_link: /docs/concepts/storage/volumes/#csi
+short_description: >
+    The Container Storage Interface (CSI) defines a standard interface to expose storage systems to containers.
+
+
+aka: 
+tags:
+- storage 
+---
+-->
+<!--
+ The Container Storage Interface (CSI) defines a standard interface to expose storage systems to containers.
+-->
+
+ 容器存储接口 （CSI） 定义了存储系统暴露给容器的标准接口。
+
+<!--more--> 
+
+<!--
+CSI allows vendors to create custom storage plugins for Kubernetes without adding them to the Kubernetes repository (out-of-tree plugins). To use a CSI driver from a storage provider, you must first [deploy it to your cluster](https://kubernetes-csi.github.io/docs/deploying.html). You will then be able to create a {{< glossary_tooltip text="Storage Class" term_id="storage-class" >}} that uses that CSI driver.
+
+* [CSI in the Kubernetes documentation](/docs/concepts/storage/volumes/#csi)
+* [List of available CSI drivers](https://kubernetes-csi.github.io/docs/drivers.html)
+-->
+
+CSI 允许存储驱动提供商为 Kubernetes 创建定制化的存储插件，
+而无需将这些插件的代码添加到 Kubernetes 代码仓库（外部插件）。
+要使用某个存储提供商的 CSI 驱动，你首先要
+[将它部署到你的集群上](https://kubernetes-csi.github.io/docs/deploying.html)。
+然后你才能创建使用该 CSI 驱动的 {{< glossary_tooltip text="Storage Class" term_id="storage-class" >}} 。
+
+* [Kubernetes 文档中关于 CSI 的描述](/zh/docs/concepts/storage/volumes/#csi)
+* [可用的 CSI 驱动列表](https://kubernetes-csi.github.io/docs/drivers.html)