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/approver.md

@@ -0,0 +1,42 @@
+---
+title: 批准者（Approver）
+id: approver
+date: 2018-04-12
+full_link: 
+short_description: >
+  可以审核并批准 Kubernetes 代码贡献的人。
+
+aka: 
+tags:
+- community
+---
+
+<!--
+---
+title: Approver
+id: approver
+date: 2018-04-12
+full_link: 
+short_description: >
+  A person who can review and approve Kubernetes code contributions.
+
+aka: 
+tags:
+- community
+---
+-->
+
+<!--
+ A person who can review and approve Kubernetes code contributions.
+-->
+ 可以审核并批准 Kubernetes 代码贡献的人。
+
+<!--more--> 
+
+<!--
+While code review is focused on code quality and correctness, approval is focused on the holistic acceptance of a contribution. Holistic acceptance includes backwards/forwards compatibility, adhering to API and flag conventions, subtle performance and correctness issues, interactions with other parts of the system, and others. Approver status is scoped to a part of the codebase. Approvers were previously referred to as maintainers.
+-->
+
+代码审核的重点是代码质量和正确性，而批准的重点是对贡献的整体接受。
+整体接受包括向后/向前兼容性、遵守 API 和参数约定、细微的性能和正确性问题、与系统其他部分的交互等。
+批准者状态的作用域是代码库的一部分。审批者以前被称为维护者。