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/uid.md

@@ -0,0 +1,43 @@
+---
+title: UID
+id: uid
+date: 2018-04-12
+full_link: /zh/docs/concepts/overview/working-with-objects/names/
+short_description: >
+  由 Kubernetes 系统生成、用来唯一标识对象的字符串。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+---
+title: UID
+id: uid
+date: 2018-04-12
+full_link: /docs/concepts/overview/working-with-objects/names
+short_description: >
+  A Kubernetes systems-generated string to uniquely identify objects.
+
+aka: 
+tags:
+- fundamental
+---
+-->
+
+<!--
+ A Kubernetes systems-generated string to uniquely identify objects.
+-->
+
+Kubernetes 系统生成的字符串，唯一标识对象。
+
+<!--more--> 
+
+<!--
+Every object created over the whole lifetime of a Kubernetes cluster has a distinct UID. It is intended to distinguish between historical occurrences of similar entities.
+-->
+
+在 Kubernetes 集群的整个生命周期中创建的每个对象都有一个不同的 uid，它旨在区分类似实体的历史事件。
+
+