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/statefulset.md

@@ -0,0 +1,55 @@
+---
+title: StatefulSet
+id: statefulset
+date: 2018-04-12
+full_link: /zh/docs/concepts/workloads/controllers/statefulset/
+short_description: >
+  StatefulSet 用来管理某 Pod 集合的部署和扩缩，并为这些 Pod 提供持久存储和持久标识符。
+aka: 
+tags:
+- fundamental
+- core-object
+- workload
+- storage
+---
+
+<!--
+---
+title: StatefulSet
+id: statefulset
+date: 2018-04-12
+full_link: /docs/concepts/workloads/controllers/statefulset/
+short_description: >
+  Manages deployment and scaling of a set of Pods, with durable storage and persistent identifiers for each Pod.
+
+aka: 
+tags:
+- fundamental
+- core-object
+- workload
+- storage
+---
+-->
+
+<!--
+ Manages the deployment and scaling of a set of {{< glossary_tooltip text="Pods" term_id="pod" >}}, *and provides guarantees about the ordering and uniqueness* of these Pods.
+-->
+StatefulSet 用来管理某 {{< glossary_tooltip text="Pod" term_id="pod" >}} 集合的部署和扩缩，
+并为这些 Pod 提供持久存储和持久标识符。 
+<!--more--> 
+
+<!--
+Like a {{< glossary_tooltip term_id="deployment" >}}, a StatefulSet manages Pods that are based on an identical container spec. Unlike a Deployment, a StatefulSet maintains a sticky identity for each of their Pods. These pods are created from the same spec, but are not interchangeable&#58; each has a persistent identifier that it maintains across any rescheduling.
+-->
+
+和 {{< glossary_tooltip text="Deployment" term_id="deployment" >}} 类似，
+StatefulSet 管理基于相同容器规约的一组 Pod。但和 Deployment 不同的是，
+StatefulSet 为它们的每个 Pod 维护了一个有粘性的 ID。这些 Pod 是基于相同的规约来创建的，
+但是不能相互替换：无论怎么调度，每个 Pod 都有一个永久不变的 ID。
+<!--
+If you want to use storage volumes to provide persistence for your workload, you can use a StatefulSet as part of the solution. Although individual Pods in a StatefulSet are susceptible to failure, the persistent Pod identifiers make it easier to match existing volumes to the new Pods that replace any that have failed.
+-->
+
+如果希望使用存储卷为工作负载提供持久存储，可以使用 StatefulSet 作为解决方案的一部分。
+尽管 StatefulSet 中的单个 Pod 仍可能出现故障，
+但持久的 Pod 标识符使得将现有卷与替换已失败 Pod 的新 Pod 相匹配变得更加容易。