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/container-runtime.md

@@ -0,0 +1,49 @@
+---
+title: 容器运行时（Container Runtime）
+id: container-runtime
+date: 2019-06-05
+full_link: /zh/docs/setup/production-environment/container-runtimes
+short_description: >
+ 容器运行时是负责运行容器的软件。
+
+aka:
+tags:
+- fundamental
+- workload
+---
+<!--
+---
+title: Container Runtime
+id: container-runtime
+date: 2019-06-05
+full_link: /docs/setup/production-environment/container-runtimes
+short_description: >
+ The container runtime is the software that is responsible for running containers.
+
+aka:
+tags:
+- fundamental
+- workload
+---
+-->
+
+<!--
+ The container runtime is the software that is responsible for running containers.
+-->
+容器运行环境是负责运行容器的软件。
+
+<!--more-->
+
+<!--
+Kubernetes supports container runtimes such sa
+{{< glossary_tooltip term_id="docker">}},
+{{< glossary_tooltip term_id="containerd" >}}, {{< glossary_tooltip term_id="cri-o" >}},
+and any other implementation of the [Kubernetes CRI (Container Runtime
+Interface)](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/container-runtime-interface.md).
+-->
+Kubernetes 支持许多容器运行环境，例如
+{{< glossary_tooltip term_id="docker">}}、
+{{< glossary_tooltip term_id="containerd" >}}、
+{{< glossary_tooltip term_id="cri-o" >}}
+以及 [Kubernetes CRI (容器运行环境接口)](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/container-runtime-interface.md)
+的其他任何实现。