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/reviewer.md

@@ -0,0 +1,41 @@
+---
+title: 评审者（Reviewer）
+id: reviewer
+date: 2018-04-12
+full_link: 
+short_description: >
+  评审者是负责评审项目的某部分代码以便提高代码质量和正确性的人。
+
+aka: 
+tags:
+- community
+---
+
+<!--
+---
+title: Reviewer
+id: reviewer
+date: 2018-04-12
+full_link: 
+short_description: >
+  A person who reviews code for quality and correctness on some part of the project.
+
+aka: 
+tags:
+- community
+---
+-->
+
+<!--
+ A person who reviews code for quality and correctness on some part of the project.
+-->
+
+ 评审者是负责评审项目的某部分代码以便提高代码质量和正确性的人。
+
+<!--more--> 
+
+<!--
+Reviewers are knowledgeable about both the codebase and software engineering principles. Reviewer status is scoped to a part of the codebase.
+-->
+
+评审者既要了解代码库又要了解软件工程规范。评审者状态是基于代码库的组成部分来设定的。