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/wg.md

@@ -0,0 +1,46 @@
+---
+title: 工作组（Working Group，WG）
+id: wg
+date: 2018-04-12
+full_link: https://github.com/kubernetes/community/blob/master/sig-list.md#master-working-group-list
+short_description: >
+  工作组是为了方便讨论和（或）推进执行一些短周期、窄范围、或者从委员会和 SIG 分离出来的项目、以及跨 SIG 的活动。
+ 
+aka: 
+tags:
+- community
+---
+
+<!--
+---
+title: WG (working group)
+id: wg
+date: 2018-04-12
+full_link: https://github.com/kubernetes/community/blob/master/sig-list.md#master-working-group-list
+short_description: >
+  Facilitates the discussion and/or implementation of a short-lived, narrow, or decoupled project for a committee, SIG, or cross-SIG effort.
+
+aka: 
+tags:
+- community
+---
+-->
+
+
+<!--
+ Facilitates the discussion and/or implementation of a short-lived, narrow, or decoupled project for a committee, {{< glossary_tooltip text="SIG" term_id="sig" >}}, or cross-SIG effort.
+-->
+
+工作组是为了方便讨论和（或）推进执行一些短周期、窄范围、或者从委员会和 SIG 分离出来的项目、以及跨 SIG 的活动。
+
+<!--more--> 
+
+<!--
+Working groups are a way of organizing people to accomplish a discrete task.
+
+For more information, see the [kubernetes/community](https://github.com/kubernetes/community) repo and the current list of [SIGs and working groups](https://github.com/kubernetes/community/blob/master/sig-list.md).
+-->
+
+工作组可以将人们组织起来，一起完成一项分散的任务。
+
+更多信息请参考 [kubernetes/community](https://github.com/kubernetes/community) 代码库和当前的 [SIGs 和工作组](https://github.com/kubernetes/community/blob/master/sig-list.md) 列表。