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/replica-set.md

@@ -0,0 +1,45 @@
+---
+title: ReplicaSet
+id: replica-set
+date: 2018-04-12
+full_link: /zh/docs/concepts/workloads/controllers/replicaset/
+short_description: >
+  ReplicaSet 是下一代副本控制器。
+
+aka: 
+tags:
+- fundamental
+- core-object
+- workload
+---
+
+<!--
+---
+title: ReplicaSet
+id: replica-set
+date: 2018-04-12
+full_link: /zh/docs/concepts/workloads/controllers/replicaset/
+short_description: >
+  ReplicaSet is the next-generation Replication Controller.
+
+aka: 
+tags:
+- fundamental
+- core-object
+- workload
+---
+-->
+
+<!--
+ ReplicaSet is the next-generation Replication Controller.
+-->
+
+ReplicaSet 是下一代副本控制器。
+
+<!--more--> 
+
+<!--
+ReplicaSet, like ReplicationController, ensures that a specified number of pods replicas are running at one time. ReplicaSet supports the new set-based selector requirements as described in the labels user guide, whereas a Replication Controller only supports equality-based selector requirements.
+-->
+
+ReplicaSet 就像 ReplicationController 那样，确保一次运行指定数量的 Pod 副本。ReplicaSet 支持新的基于集合的选择器需求（在标签的用户指南中有相关描述），而副本控制器只支持基于等值的选择器需求。