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/cronjob.md

@@ -0,0 +1,43 @@
+---
+title: 周期调度任务（CronJob）
+id: cronjob
+date: 2018-04-12
+full_link: /zh/docs/concepts/workloads/controllers/cron-jobs/
+short_description: >
+  周期调度的任务（作业）。
+
+aka: 
+tags:
+- core-object
+- workload
+---
+
+<!--
+---
+title: CronJob
+id: cronjob
+date: 2018-04-12
+full_link: /docs/concepts/workloads/controllers/cron-jobs/
+short_description: >
+  A repeating task (a Job) that runs on a regular schedule.
+
+aka: 
+tags:
+- core-object
+- workload
+---
+-->
+
+<!--
+ Manages a [Job](/docs/concepts/workloads/controllers/job/) that runs on a periodic schedule.
+-->
+
+ 管理定期运行的 [任务](/zh/docs/concepts/workloads/controllers/job/)。
+
+<!--more--> 
+
+<!--
+Similar to a line in a *crontab* file, a Cronjob object specifies a schedule using the [Cron](https://en.wikipedia.org/wiki/Cron) format.
+-->
+
+Cronjob 对象类似 *crontab* 文件中的一行命令，它声明了一个遵循 [Cron](https://en.wikipedia.org/wiki/Cron) 格式的调度任务。