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/device-plugin.md

@@ -0,0 +1,52 @@
+---
+title: 设备插件（Device Plugin）
+id: device-plugin
+date: 2019-02-02
+full_link: /zh/docs/concepts/extend-kubernetes/compute-storage-net/device-plugins/
+short_description: >
+  一种软件扩展，可以使 Pod 访问由特定厂商初始化或者安装的设备。
+aka:
+tags:
+- fundamental
+- extension
+---
+
+<!-- 
+---
+title: Device Plugin
+id: device-plugin
+date: 2019-02-02
+full_link: /docs/concepts/extend-kubernetes/compute-storage-net/device-plugins/
+short_description: >
+  Software extensions to let Pods access devices that need vendor-specific initialization or setup
+aka:
+tags:
+- fundamental
+- extension
+---
+-->
+<!-- 
+ Device plugins run on worker
+{{< glossary_tooltip term_id="node" text="Nodes">}} and provide
+{{< glossary_tooltip term_id="pod" text="Pods ">}} with access to resources,
+such as local hardware, that require vendor-specific initialization or setup
+steps.
+-->
+设备插件工作在节点主机上，给 {{< glossary_tooltip term_id="pod" text="Pods ">}} 提供访问资源的权限，比如特定厂商初始化或者安装的本地硬件。
+<!--more-->
+
+<!--
+Device plugins advertise resources to the
+{{< glossary_tooltip term_id="kubelet" text="kubelet" >}}, so that workload
+Pods can access hardware features that relate to the Node where that Pod is running.
+You can deploy a device plugin as a {{< glossary_tooltip term_id="daemonset" >}},
+or install the device plugin software directly on each target Node.
+-->
+设备插件将资源告知 {{< glossary_tooltip term_id="kubelet" text="kubelet" >}} ，以便相关节点上运行的工作负载Pod可以访问硬件功能。
+<!-- 
+See
+	 
+[Device Plugins](/docs/concepts/extend-kubernetes/compute-storage-net/device-plugins/)
+for more information.
+-->
+更多信息请查阅[设备插件](/zh/docs/concepts/extend-kubernetes/compute-storage-net/device-plugins/)