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.md

@@ -0,0 +1,45 @@
+---
+title: 服务（Service）
+id: service
+date: 2018-04-12
+full_link: /zh/docs/concepts/services-networking/service/
+short_description: >
+  将运行在一组 Pods 上的应用程序公开为网络服务的抽象方法。
+
+aka:
+tags:
+- fundamental
+- core-object
+---
+
+<!--
+---
+title: Service
+id: service
+date: 2018-04-12
+full_link: /docs/concepts/services-networking/service/
+short_description: >
+  A way to expose an application running on a set of Pods as a network service.
+
+aka:
+tags:
+- fundamental
+- core-object
+---
+-->
+
+
+<!--
+An abstract way to expose an application running on a set of {{< glossary_tooltip text="Pods" term_id="pod" >}} as a network service.
+-->
+
+将运行在一组 {{< glossary_tooltip text="Pods" term_id="pod" >}} 上的应用程序公开为网络服务的抽象方法。
+
+<!--more-->
+
+<!--
+ The set of Pods targeted by a Service is (usually) determined by a {{< glossary_tooltip text="selector" term_id="selector" >}}. If more Pods are added or removed, the set of Pods matching the selector will change. The Service makes sure that network traffic can be directed to the current set of Pods for the workload.
+-->
+服务所针对的 Pod 集（通常）由{{< glossary_tooltip text="选择算符" term_id="selector" >}}确定。
+如果有 Pod 被添加或被删除，则与选择算符匹配的 Pod 集合将发生变化。
+服务确保可以将网络流量定向到该工作负载的当前 Pod 集合。