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/minikube.md

@@ -0,0 +1,47 @@
+---
+title: Minikube
+id: minikube
+date: 2018-04-12
+full_link: /docs/getting-started-guides/minikube/
+short_description: >
+  Minikube 是用来在本地运行 Kubernetes 的一种工具。
+
+aka: 
+tags:
+- fundamental
+- tool
+---
+
+<!--
+---
+title: Minikube
+id: minikube
+date: 2018-04-12
+full_link: /docs/getting-started-guides/minikube/
+short_description: >
+  A tool for running Kubernetes locally.
+
+aka: 
+tags:
+- fundamental
+- tool
+---
+-->
+
+<!--
+ A tool for running Kubernetes locally.
+-->
+
+Minikube 是用来在本地运行 Kubernetes 的一种工具。
+
+<!--more--> 
+
+<!--
+Minikube runs a single-node cluster inside a VM on your computer.
+You can use Minikube to
+[try Kubernetes in a learning environment](/docs/setup/learning-environment/).
+-->
+
+Minikube 在用户计算机上的一个虚拟机内运行单节点 Kubernetes 集群。
+你可以使用 Minikube 
+[在学习环境中尝试 Kubernetes](/zh/docs/setup/learning-environment/).