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/replication-controller.md

@@ -0,0 +1,53 @@
+---
+title: 副本控制器（Replication Controller）
+id: replication-controller
+date: 2018-04-12
+full_link: 
+short_description: >
+  一种管理多副本应用的（已启用）的 API 对象。
+
+aka: 
+tags:
+- workload
+- core-object
+---
+
+<!--
+title: Replication Controller
+id: replication-controller
+date: 2018-04-12
+full_link: 
+short_description: >
+  A (deprecated) API object that manages a replicated application.
+
+aka: 
+tags:
+- workload
+- core-object
+-->
+
+<!--
+ A workload resource that manages a replicated application, ensuring that
+a specific number of instances of a {{< glossary_tooltip text="Pod" term_id="pod" >}} are running.
+-->
+一种工作管理多副本应用的负载资源，能够确保特定个数的
+{{< glossary_tooltip text="Pod" term_id="pod" >}}
+实例处于运行状态。
+
+<!--more--> 
+
+<!--
+The control plane ensures that the defined number of Pods are running, even if some
+Pods fail, if you delete Pods manually, or if too many are started by mistake.
+-->
+控制面确保所指定的个数的 Pods 处于运行状态，即使某些 Pod 会失效，
+比如被你手动删除或者因为其他错误启动过多 Pod 时。
+
+{{< note >}}
+<!--
+ReplicationController is deprecated. See
+{{< glossary_tooltip text="Deployment" term_id="deployment" >}}, which is similar.
+-->
+ReplicationController 已被启用。请参见 Deployment 执行类似功能。
+{{< /note >}}
+