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/preemption.md

@@ -0,0 +1,42 @@
+---
+title: 抢占（Preemption）
+id: preemption
+date: 2019-01-31
+full_link: /zh/docs/concepts/scheduling-eviction/pod-priority-preemption/#preemption
+short_description: >
+  Kubernetes 中的抢占逻辑通过驱逐节点上的低优先级 Pod 来帮助悬决的
+  Pod 找到合适的节点。
+
+aka:
+tags:
+- operation
+---
+
+<!--
+title: Preemption
+id: preemption
+date: 2019-01-31
+full_link: /docs/concepts/scheduling-eviction/pod-priority-preemption/#preemption
+short_description: >
+  Preemption logic in Kubernetes helps a pending Pod to find a suitable Node by evicting low priority Pods existing on that Node.
+
+aka:
+tags:
+- operation
+-->
+<!--
+ Preemption logic in Kubernetes helps a pending {{< glossary_tooltip term_id="pod" >}} to find a suitable {{< glossary_tooltip term_id="node" >}} by evicting low priority Pods existing on that Node.
+-->
+Kubernetes 中的抢占逻辑通过驱逐{{< glossary_tooltip term_id="node" >}}
+上的低优先级{{< glossary_tooltip term_id="pod" >}}
+来帮助悬决的 Pod 找到合适的节点。
+
+<!--more-->
+
+<!--
+If a Pod cannot be scheduled, the scheduler tries to [preempt](/docs/concepts/scheduling-eviction/pod-priority-preemption/#preemption) lower priority Pods to make scheduling of the pending Pod possible.
+-->
+如果一个 Pod 无法调度，调度器会尝试
+[抢占](/zh/docs/concepts/scheduling-eviction/pod-priority-preemption/#preemption)
+较低优先级的 Pod，以使得悬决的 Pod 有可能被调度。
+