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/volume.md

@@ -0,0 +1,50 @@
+---
+title: 卷（Volume）
+id: volume
+date: 2018-04-12
+full_link: /zh/docs/concepts/storage/volumes/
+short_description: >
+  包含可被 Pod 中容器访问的数据的目录。
+
+aka: 
+tags:
+- core-object
+- fundamental
+---
+
+<!--
+---
+title: Volume
+id: volume
+date: 2018-04-12
+full_link: /docs/concepts/storage/volumes/
+short_description: >
+  A directory containing data, accessible to the containers in a pod.
+
+aka:
+tags:
+- core-object
+- fundamental
+---																																					   
+-->
+
+
+<!--
+ A directory containing data, accessible to the {{< glossary_tooltip text="containers" term_id="container" >}} in a {{< glossary_tooltip term_id="pod" >}}.
+-->
+包含可被 {{< glossary_tooltip text="Pod" term_id="pod" >}} 中{{< glossary_tooltip text="容器" term_id="container" >}}访问的数据的目录。
+
+		   
+
+<!--
+A Kubernetes volume lives as long as the Pod that encloses it. Consequently, a volume outlives any containers that run within the Pod, and data in the volume is preserved across container restarts.
+-->
+
+每个 Kubernetes 卷在所处的 {{< glossary_tooltip text="Pod" term_id="pod" >}} 存在期间保持存在状态。
+因此，卷的生命期会超出 {{< glossary_tooltip text="Pod" term_id="pod" >}} 中运行的{{< glossary_tooltip text="容器" term_id="container" >}}，
+并且保证{{< glossary_tooltip text="容器" term_id="container" >}}重启之后仍保留数据。
+
+<!--
+See [storage](/docs/concepts/storage/) for more information.
+-->
+更多信息可参考[storage](/zh/docs/concepts/storage/)