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/mirror-pod.md

@@ -0,0 +1,47 @@
+---
+title: 镜像 Pod（Mirror Pod）
+id: 静态-pod
+date: 2019-08-06
+short_description: >
+  API 服务器中的一个对象，用于跟踪 kubelet 上的静态 pod。
+
+aka:
+tags:
+- 基本的
+---
+<!--
+---
+title: Mirror Pod
+id: mirror-pod
+date: 2019-08-06
+short_description: >
+  An object in the API server that tracks a static pod on a kubelet.
+
+aka:
+tags:
+- fundamental
+---
+-->
+
+<!--
+ A {{< glossary_tooltip text="pod" term_id="pod" >}} object that a kubelet uses
+ to represent a {{< glossary_tooltip text="static pod" term_id="static-pod" >}}
+-->
+镜像 Pod（Mirror Pod）是被 kubelet 用来代表{{< glossary_tooltip text="静态 Pod" term_id="static-pod" >}} 的
+{{< glossary_tooltip text="pod" term_id="pod" >}} 对象。
+
+<!--more-->
+<!--更多-->
+
+<!--
+When the kubelet finds a static pod in its configuration, it automatically tries to
+create a Pod object on the Kubernetes API server for it. This means that the pod
+will be visible on the API server, but cannot be controlled from there.
+
+(For example, removing a mirror pod will not stop the kubelet daemon from running it).
+-->
+当 kubelet 在其配置中发现一个静态容器时，
+它会自动地尝试在 Kubernetes API 服务器上为它创建 Pod 对象。
+这意味着 pod 在 API 服务器上将是可见的，但不能在其上进行控制。
+
+（例如，删除镜像 Pod 也不会阻止 kubelet 守护进程继续运行它）。