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/cni.md

@@ -0,0 +1,44 @@
+---
+title: 容器网络接口（CNI）
+id: cni
+date: 2018-05-25
+full_link: /zh/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/#cni
+short_description: >
+    容器网络接口 (CNI) 插件是遵循 appc/CNI 协议的一类网络插件。
+
+
+aka: 
+tags:
+- networking 
+---
+
+<!--
+---
+title: Container network interface (CNI)
+id: cni
+date: 2018-05-25
+full_link: /docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/#cni
+short_description: >
+    Container network interface (CNI) plugins are a type of Network plugin that adheres to the appc/CNI specification.
+
+
+aka: 
+tags:
+- networking 
+---
+-->
+
+<!--
+ Container network interface (CNI) plugins are a type of Network plugin that adheres to the appc/CNI specification.
+-->
+
+ 容器网络接口 (CNI) 插件是遵循 appc/CNI 协议的一类网络插件。
+
+<!--more--> 
+
+<!--
+* For information on Kubernetes and CNI refer to [this](/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/#cni).
+* For information on Kubernetes and CNI, see ["Network plugins"](/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/#cni).
+-->
+
+* 想了解 Kubernetes 和 CNI 请参考 ["网络插件"](/zh/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/#cni)。