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/label.md

@@ -0,0 +1,41 @@
+---
+title: 标签（Label）
+id: label
+date: 2018-04-12
+full_link: /zh/docs/concepts/overview/working-with-objects/labels/
+short_description: >
+  用来为对象设置可标识的属性标记；这些标记对用户而言是有意义且重要的。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+title: Label
+id: label
+date: 2018-04-12
+full_link: /docs/concepts/overview/working-with-objects/labels/
+short_description: >
+  Tags objects with identifying attributes that are meaningful and relevant to users.
+
+aka: 
+tags:
+- fundamental
+-->
+
+<!--
+ Tags objects with identifying attributes that are meaningful and relevant to users.
+-->
+
+用来为对象设置可标识的属性标记；这些标记对用户而言是有意义且重要的。
+
+<!--more--> 
+
+<!--
+Labels are key/value pairs that are attached to objects such as {{< glossary_tooltip text="Pods" term_id="pod" >}}. They are used to organize and to select subsets of objects.
+-->
+
+标签是一些关联到 {{< glossary_tooltip text="Pods" term_id="pod" >}} 这类对象上的键值对。
+它们通常用来组织和选择对象子集。
+