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/toleration.md

@@ -0,0 +1,49 @@
+---
+title: 容忍度（Toleration）
+id: toleration
+date: 2019-01-11
+full_link: /zh/docs/concepts/scheduling-eviction/taint-and-toleration/
+short_description: >
+  一个核心对象，由三个必需的属性组成：key、value 和 effect。
+  容忍度允许将 Pod 调度到具有对应污点的节点或节点组上。
+
+aka:
+tags:
+- core-object
+- fundamental
+---
+
+<!--
+title: Toleration
+id: toleration
+date: 2019-01-11
+full_link: /docs/concepts/scheduling-eviction/taint-and-toleration/
+short_description: >
+  A core object consisting of three required properties: key, value, and effect. Tolerations enable the scheduling of pods on nodes or node groups that have a matching taint.
+  
+aka:
+tags:
+- core-object
+- fundamental
+-->
+
+<!--
+ A core object consisting of three required properties: key, value, and effect. Tolerations enable the scheduling of pods on nodes or node groups that have matching {{< glossary_tooltip text="taints" term_id="taint" >}}.
+-->
+一个核心对象，由三个必需的属性组成：key、value 和 effect。
+容忍度允许将 Pod 调度到具有对应{{< glossary_tooltip text="污点" term_id="taint" >}}
+的节点或节点组上。
+ 
+<!--more-->
+
+<!--
+Tolerations and {{< glossary_tooltip text="taints" term_id="taint" >}} work together to ensure that pods are not scheduled onto inappropriate nodes. One or more tolerations are applied to a {{< glossary_tooltip text="pod" term_id="pod" >}}. A toleration indicates that the {{< glossary_tooltip text="pod" term_id="pod" >}} is allowed (but not required) to be scheduled on nodes or node groups with matching {{< glossary_tooltip text="taints" term_id="taint" >}}.
+-->
+容忍度和{{< glossary_tooltip text="污点" term_id="taint" >}}共同作用可以
+确保不会将 Pod 调度在不适合的节点上。
+在同一 {{< glossary_tooltip text="Pod" term_id="pod" >}} 上可以设置一个
+或者多个容忍度。
+容忍度表示在包含对应{{< glossary_tooltip text="污点" term_id="taint" >}}
+的节点或节点组上调度 {{< glossary_tooltip text="Pod" term_id="pod" >}}
+是允许的（但不必要）。
+