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/kubelet.md

@@ -0,0 +1,43 @@
+---
+title: Kubelet
+id: kubelet
+date: 2018-04-12
+full_link: /docs/reference/generated/kubelet
+short_description: >
+  一个在集群中每个节点上运行的代理。它保证容器都运行在 Pod 中。
+
+aka: 
+tags:
+- fundamental
+- core-object
+---
+<!--
+title: Kubelet
+id: kubelet
+date: 2018-04-12
+full_link: /docs/reference/generated/kubelet
+short_description: >
+  An agent that runs on each node in the cluster. It makes sure that containers are running in a pod.
+
+aka: 
+tags:
+- fundamental
+- core-object
+-->
+
+<!--
+ An agent that runs on each {{< glossary_tooltip text="node" term_id="node" >}} in the cluster. It makes sure that {{< glossary_tooltip text="containers" term_id="container" >}} are running in a {{< glossary_tooltip text="Pod" term_id="pod" >}}.
+-->
+`kubelet` 会在集群中每个{{< glossary_tooltip text="节点（node）" term_id="node" >}}上运行。
+它保证{{< glossary_tooltip text="容器（containers）" term_id="container" >}}都运行在
+{{< glossary_tooltip text="Pod" term_id="pod" >}} 中。
+
+<!--more--> 
+
+<!--
+The kubelet takes a set of PodSpecs that are provided through various mechanisms and ensures that the containers described in those PodSpecs are running and healthy. The kubelet doesn't manage containers which were not created by Kubernetes.
+-->
+kubelet 接收一组通过各类机制提供给它的 PodSpecs，
+确保这些 PodSpecs 中描述的容器处于运行状态且健康。
+kubelet 不会管理不是由 Kubernetes 创建的容器。
+