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/selector.md

@@ -0,0 +1,44 @@
+---
+title: 选择算符（Selector）
+id: selector
+date: 2018-04-12
+full_link: /zh/docs/concepts/overview/working-with-objects/labels/
+short_description: >
+  选择算符允许用户通过标签对一组资源对象进行筛选过滤。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+
+---
+title: Selector
+id: selector
+date: 2018-04-12
+full_link: /docs/concepts/overview/working-with-objects/labels/
+short_description: >
+  Allows users to filter a list of resources based on labels.
+
+aka: 
+tags:
+- fundamental
+---
+-->
+
+
+<!--
+ Allows users to filter a list of resources based on {{< glossary_tooltip text="labels" term_id="label" >}}.
+-->
+
+选择算符允许用户通过{{< glossary_tooltip text="标签（labels）" term_id="label" >}}对一组资源对象进行筛选过滤。
+
+<!--more--> 
+
+<!--
+Selectors are applied when querying lists of resources to filter them by labels.
+-->
+
+在查询资源列表时，选择算符可以通过标签对资源进行过滤筛选。
+