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/pod-disruption-budget.md

@@ -0,0 +1,49 @@
+---
+id: pod-disruption-budget
+title: Pod Disruption Budget
+full-link: /zh/docs/concepts/workloads/pods/disruptions/
+date: 2019-02-12
+short_description: >
+ Pod Disruption Budget 是这样一种对象：它保证在主动中断（ voluntary disruptions）时，多实例应用的 {{< glossary_tooltip text="Pod" term_id="pod" >}} 不会少于一定的数量。
+
+aka:
+ - PDB
+related:
+ - pod
+ - container
+tags:
+ - operation
+---
+
+<!--
+---
+id: pod-disruption-budget
+title: Pod Disruption Budget
+full-link: /docs/concepts/workloads/pods/disruptions/
+date: 2019-02-12
+short_description: >
+ An object that limits the number of {{< glossary_tooltip text="Pods" term_id="pod" >}} of a replicated application, that are down simultaneously from voluntary disruptions.
+
+aka:
+ - PDB
+related:
+ - pod
+ - container
+tags:
+ - operation
+---
+-->
+
+<!--
+ A [Pod Disruption Budget](/docs/concepts/workloads/pods/disruptions/) allows an 
+ application owner to create an object for a replicated application, that ensures
+ a certain number or percentage of Pods with an assigned label will not be voluntarily
+ evicted at any point in time.
+
+ Involuntary disruptions cannot be prevented by PDBs; however they 
+ do count against the budget.
+-->
+ [Pod 干扰预算（Pod Disruption Budget，PDB）](/zh/docs/concepts/workloads/pods/disruptions/)
+ 使应用所有者能够为多实例应用创建一个对象，来确保一定数量的具有指定标签的 Pod 在任何时候都不会被主动驱逐。
+ <!--more--> 
+PDB 无法防止非主动的中断，但是会计入预算（budget）。