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/deployment.md

@@ -0,0 +1,48 @@
+---
+title: Deployment
+id: deployment
+date: 2018-04-12
+full_link: /zh/docs/concepts/workloads/controllers/deployment/
+short_description: >
+  Deployment 是管理应用副本的 API 对象。
+
+aka: 
+tags:
+- fundamental
+- core-object
+- workload
+---
+
+<!--
+---
+title: Deployment
+id: deployment
+date: 2018-04-12
+full_link: /docs/concepts/workloads/controllers/deployment/
+short_description: >
+  Manages a replicated application on your cluster.
+
+aka: 
+tags:
+- fundamental
+- core-object
+- workload
+---
+-->
+
+<!--
+ An API object that manages a replicated application, typically by running Pods with no local state.
+-->
+
+ Deployment 是管理应用副本的 API 对象，通常通过运行没有本地状态的Pods来实现。
+
+<!--more--> 
+
+<!--
+Each replica is represented by a {{< glossary_tooltip term_id="pod" >}}, and the Pods are distributed among the 
+{{< glossary_tooltip text="nodes" term_id="node" >}} of a cluster.
+For workloads that do require local state, consider using a {{< glossary_tooltip term_id="StatefulSet" >}}.
+-->
+
+应用的每个副本就是一个 {{< glossary_tooltip text="Pod" term_id="pod" >}}，
+并且这些 Pod 会分散运行在集群的{{< glossary_tooltip text="节点" term_id="node" >}}上。