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/upstream.md

@@ -0,0 +1,42 @@
+---
+title: 上游（Uptream）
+id: upstream
+date: 2018-04-12
+full_link: 
+short_description: >
+  可以参考：核心 Kubernetes 仓库或作为当前仓库派生来源的来源仓库。
+
+aka: 
+tags:
+- community
+---
+
+<!--
+---
+title: Upstream (disambiguation)
+id: upstream
+date: 2018-04-12
+full_link: 
+short_description: >
+  May refer to: core Kubernetes or the source repo from which a repo was forked.
+
+aka: 
+tags:
+- community
+---
+-->
+
+<!--
+ May refer to: core Kubernetes or the source repo from which a repo was forked.
+-->
+可能指的是：核心 Kubernetes 仓库或作为当前仓库派生来源的仓库。
+
+<!--more--> 
+
+<!--
+* In the **Kubernetes Community**: Conversations often use *upstream* to mean the core Kubernetes codebase, which the general ecosystem, other code, or third-party tools relies upon. For example, [community members](#term-member) may suggest that a feature is moved upstream so that it is in the core codebase instead of in a plugin or third-party tool.
+* In **GitHub** or **git**: The convention is to refer to a source repo as *upstream*, whereas the forked repo is considered *downstream*.
+-->
+
+* 在 **Kubernetes社区**：对话中通常使用 *upstream* 来表示核心 Kubernetes 代码库，也就是更广泛的 kubernetes 生态系统、其他代码或第三方工具所依赖的仓库。 例如，[社区成员](#term-member)可能会建议将某个功能特性贡献到 upstream，使其位于核心代码库中，而不是维护于插件或第三方工具中。
+* 在 **GitHub** 或 **git** 中：约定是将源仓库称为 *upstream*，而派生的仓库则被视为 *downstream*。