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-catalog.md

@@ -0,0 +1,45 @@
+---
+title: 服务目录（Service Catalog）
+id: service-catalog
+date: 2018-04-12
+full_link: 
+short_description: >
+  服务目录是一种扩展 API，它能让 Kubernetes 集群中运行的应用易于使用外部托管的软件服务，例如云供应商提供的数据仓库服务。
+  
+aka: 
+tags:
+- extension
+---
+
+<!--
+---
+title: Service Catalog
+id: service-catalog
+date: 2018-04-12
+full_link: 
+short_description: >
+  An extension API that enables applications running in Kubernetes clusters to easily use external managed software offerings, such as a datastore service offered by a cloud provider.
+
+aka: 
+tags:
+- extension
+---
+-->
+
+
+<!--
+ An extension API that enables applications running in Kubernetes clusters to easily use external managed software offerings, such as a datastore service offered by a cloud provider.
+-->
+
+服务目录是一种扩展 API，它能让 Kubernetes 集群中运行的应用易于使用外部托管的的软件服务，例如云供应商提供的数据仓库服务。
+
+<!--more--> 
+
+<!--
+It provides a way to list, provision, and bind with external {{< glossary_tooltip text="Managed Services" term_id="managed-service" >}} from {{< glossary_tooltip text="Service Brokers" term_id="service-broker" >}} without needing detailed knowledge about how those services are created or managed.
+-->
+
+服务目录可以检索、供应、和绑定由 {{< glossary_tooltip text="服务代理人（Service Brokers）" term_id="service-broker" >}}
+提供的外部{{< glossary_tooltip text="托管服务（Managed Services）" term_id="managed-service" >}}，
+而无需知道那些服务具体是怎样创建和托管的。
+