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/logging.md

@@ -0,0 +1,39 @@
+---
+title: 日志（Logging）
+id: logging
+date: 2019-04-04
+full_link: /zh/docs/concepts/cluster-administration/logging/
+short_description: >
+  日志是集群或应用程序记录的事件列表。
+
+aka: 
+tags:
+- architecture
+- fundamental
+---
+  日志是 {{< glossary_tooltip text="集群（cluster）" term_id="cluster" >}} 或应用程序记录的事件列表。
+  
+<!--
+---
+title: Logging
+id: logging
+date: 2019-04-04
+full_link: /zh/docs/concepts/cluster-administration/logging/
+short_description: >
+  Logs are the list of events that are logged by cluster or application.
+
+aka: 
+tags:
+- architecture
+- fundamental
+---
+ Logs are the list of events that are logged by {{< glossary_tooltip text="cluster" term_id="cluster" >}} or application.
+-->
+
+<!--more--> 
+
+<!--
+Application and systems logs can help you understand what is happening inside your cluster. The logs are particularly useful for debugging problems and monitoring cluster activity. 
+-->
+
+应用程序和系统日志可以帮助你了解集群内部发生的情况。日志对于调试问题和监视集群活动非常有用。