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/service-account.md

@@ -0,0 +1,45 @@
+---
+title: ServiceAccount
+id: service-account
+date: 2018-04-12
+full_link: /zh/docs/tasks/configure-pod-container/configure-service-account/
+short_description: >
+  为在 Pod 中运行的进程提供标识。
+
+aka: 
+tags:
+- fundamental
+- core-object
+---
+
+<!--
+---
+title: ServiceAccount
+id: service-account
+date: 2018-04-12
+full_link: /docs/tasks/configure-pod-container/configure-service-account/
+short_description: >
+  Provides an identity for processes that run in a Pod.
+
+aka: 
+tags:
+- fundamental
+- core-object
+---
+-->
+
+
+<!--
+ Provides an identity for processes that run in a {{< glossary_tooltip text="Pod" term_id="pod" >}}.
+-->
+为在 {{< glossary_tooltip text="Pod" term_id="pod" >}} 中运行的进程提供标识。
+
+<!--more--> 
+
+<!--
+When processes inside Pods access the cluster, they are authenticated by the API server as a particular service account, for example, `default`. When you create a Pod, if you do not specify a service account, it is automatically assigned the default service account in the same {{< glossary_tooltip text="Namespace" term_id="namespace" >}}.
+-->
+当 Pod 中的进程访问集群时，API 服务器将它们作为特定的服务帐户进行身份验证，
+例如  `default` ，创建 Pod 时，如果你没有指定服务帐户，它将自动被赋予同一个
+{{< glossary_tooltip text="名字空间" term_id="namespace" >}}中的 default 服务账户。
+