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/downstream.md

@@ -0,0 +1,44 @@
+---
+title: 下游（Downstream）
+id: downstream
+date: 2018-04-12
+full_link: 
+short_description: >
+  可以指：Kubernetes 生态系统中依赖于核心 Kubernetes 代码库或分支代码库的代码。
+
+aka: 
+tags:
+- community
+---
+
+<!--
+---
+title: Downstream (disambiguation)
+id: downstream
+date: 2018-04-12
+full_link: 
+short_description: >
+  May refer to: code in the Kubernetes ecosystem that depends upon the core Kubernetes codebase or a forked repo.
+
+aka: 
+tags:
+- community
+---
+-->
+
+<!--
+ May refer to: code in the Kubernetes ecosystem that depends upon the core Kubernetes codebase or a forked repo.
+-->
+
+可以指：Kubernetes 生态系统中依赖于核心 Kubernetes 代码库或分支代码库的代码。
+
+<!--more--> 
+
+<!--
+* In the **Kubernetes Community**: Conversations often use *downstream* to mean the ecosystem, code, or third-party tools that rely on the core Kubernetes codebase. For example, a new feature in Kubernetes may be adopted by applications *downstream* to improve their functionality.
+* In **GitHub** or **git**: The convention is to refer to a forked repo as *downstream*, whereas the source repo is considered *upstream*.
+-->
+
+* 在 **Kubernetes 社区**中：*下游(downstream)* 在人们交流中常用来表示那些依赖核心 Kubernetes 代码库的生态系统、代码或者第三方工具。例如，Kubernetes 的一个新特性可以被*下游(downstream)* 应用采用，以提升它们的功能性。
+* 在 **GitHub** 或 **git** 中：约定用*下游(downstream)* 表示分支代码库，源代码库被认为是*上游(upstream)*。
+