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/tasks/debug/debug-application/_index.md

@@ -0,0 +1,18 @@
+---
+title: "应用故障排除"
+description: 调试常见的容器应用问题.
+weight: 20
+--- 
+
+<!-- 
+title: "Troubleshooting Applications"
+description: Debugging common containerized application issues.
+weight: 20
+-->
+
+<!-- 
+This doc contains a set of resources for fixing issues with containerized applications. It covers things like common issues with Kubernetes resources (like Pods, Services, or StatefulSets), advice on making sense of container termination messages, and ways to debug running containers.
+-->
+该文档包含一组用于解决容器化应用程序问题的资源。
+它涵盖了诸如 Kubernetes 资源（如 Pod、Service 或 StatefulSets）的常见问题、
+关于理解容器终止消息的建议以及调试正在运行的容器的方法。