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/aggregation-layer.md

@@ -0,0 +1,49 @@
+---
+title: 聚合层（Aggregation Layer）
+id: aggregation-layer
+date: 2018-10-08
+full_link: /zh/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/
+short_description: >
+  聚合层允许你在自己的集群上安装额外的 Kubernetes 风格的 API。
+
+aka: 
+tags:
+- architecture
+- extension
+- operation
+---
+<!--
+---
+title: Aggregation Layer
+id: aggregation-layer
+date: 2018-10-08
+full_link: /docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/
+short_description: >
+  The aggregation layer lets you install additional Kubernetes-style APIs in your cluster.
+
+aka: 
+tags:
+- architecture
+- extension
+- operation
+---
+-->
+
+
+
+
+<!--
+ The aggregation layer lets you install additional Kubernetes-style APIs in your cluster.
+-->
+
+聚合层允许你在自己的集群上安装额外的 Kubernetes 风格的 API。
+
+<!--more-->
+ 
+
+<!--
+When you've configured the {{< glossary_tooltip text="Kubernetes API Server" term_id="kube-apiserver" >}} to [support additional APIs](/docs/tasks/extend-kubernetes/configure-aggregation-layer/), you can add `APIService` objects to "claim" a URL path in the Kubernetes API.
+-->
+
+当你配置了 {{< glossary_tooltip text="Kubernetes API Server" term_id="kube-apiserver" >}} 来 [支持额外的 API](/zh/docs/tasks/extend-kubernetes/configure-aggregation-layer/)，
+你就可以在 Kubernetes API 中增加 `APIService` 对象来  "申领（Claim）" 一个 URL 路径。