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/qos-class.md

@@ -0,0 +1,46 @@
+---
+title: QoS 类（QoS Class）
+id: qos-class
+date: 2019-04-15
+full_link: 
+short_description: >
+ QoS 类（Quality of Service Class）为 Kubernetes 提供了一种将集群中的 Pod 分为几个类并做出有关调度和驱逐决策的方法。
+
+aka: 
+tags:
+- core-object
+- fundamental
+- architecture
+related:
+ - pod
+
+---
+<!--
+---
+title: QoS Class
+id: qos-class
+date: 2019-04-15
+full_link: 
+short_description: >
+ QoS Class (Quality of Service Class) provides a way for Kubernetes to classify pods within the cluster into several classes and make decisions about scheduling and eviction.
+aka: 
+tags:
+- core-object
+- fundamental
+- architecture
+related:
+ - pod
+
+---
+-->
+
+ QoS Class（Quality of Service Class）为 Kubernetes 提供了一种将集群中的 Pod 分为几个类型并做出有关调度和驱逐决策的方法。
+
+<!--more--> 
+
+<!--
+QoS Class of a Pod is set at creation time  based on its compute resources requests and limits settings. QoS classes are used to make decisions about Pods scheduling and eviction.
+Kubernetes can assign one of the following  QoS classes to a Pod: `Guaranteed`, `Burstable` or `BestEffort`.
+-->
+Pod 的 QoS 类是基于 Pod 在创建时配置的计算资源请求和限制。QoS 类用于制定有关 Pod 调度和逐出的决策。
+Kubernetes 可以为 Pod 分配以下 QoS 类：`Guaranteed`，`Burstable` 或者 `BestEffort`。