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/endpoint-slice.md

@@ -0,0 +1,42 @@
+---
+title: EndpointSlice
+id: endpoint-slice
+date: 2018-04-12
+full_link: /zh/docs/concepts/services-networking/endpoint-slices/
+short_description: >
+  一种将网络端点与 Kubernetes 资源组合在一起的方法。
+
+aka:
+tags:
+- networking
+---
+  一种将网络端点与 Kubernetes 资源组合在一起的方法。
+
+<!--
+---
+title: EndpointSlice
+id: endpoint-slice
+date: 2018-04-12
+full_link: /docs/concepts/services-networking/endpoint-slices/
+short_description: >
+  A way to group network endpoints together with Kubernetes resources.
+
+aka:
+tags:
+- networking
+---
+ A way to group network endpoints together with Kubernetes resources.
+-->
+
+<!--more-->
+
+<!--
+A scalable and extensible way to group network endpoints together. These can be
+used by {{< glossary_tooltip text="kube-proxy" term_id="kube-proxy" >}} to
+establish network routes on each {{< glossary_tooltip text="node" term_id="node" >}}.
+-->
+
+
+一种将网络端点组合在一起的可扩缩、可扩展方式。
+它们将被 {{< glossary_tooltip text="kube-proxy" term_id="kube-proxy" >}} 用于在
+每个 {{< glossary_tooltip text="节点" term_id="node">}} 上建立网络路由。