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-priority.md

@@ -0,0 +1,40 @@
+---
+title: Pod 优先级（Pod Priority）
+id: pod-priority
+date: 2019-01-31
+full_link: /zh/docs/concepts/scheduling-eviction/pod-priority-preemption/#pod-priority
+short_description: >
+  Pod 优先级表示一个 Pod 相对于其他 Pod 的重要性。
+
+aka:
+tags:
+- operation
+---
+
+<!--
+title: Pod Priority
+id: pod-priority
+date: 2019-01-31
+full_link: /docs/concepts/scheduling-eviction/pod-priority-preemption/#pod-priority
+short_description: >
+  Pod Priority indicates the importance of a Pod relative to other Pods.
+
+aka:
+tags:
+- operation
+-->
+
+<!--
+ Pod Priority indicates the importance of a {{< glossary_tooltip term_id="pod" >}} relative to other Pods.
+-->
+ Pod 优先级表示一个  {{< glossary_tooltip term_id="pod" >}} 相对于其他 Pod 的重要性。
+
+<!--more-->
+
+<!--
+[Pod Priority](/docs/concepts/scheduling-eviction/pod-priority-preemption/#pod-priority) gives the ability to set scheduling priority of a Pod to be higher and lower than other Pods — an important feature for production clusters workload.
+-->
+[Pod 优先级](/zh/docs/concepts/scheduling-eviction/pod-priority-preemption/#pod-priority)
+允许用户为 Pod 设置高于或低于其他 Pod 的优先级 -- 这对于生产集群
+工作负载而言是一个重要的特性。
+