Use "zh-CN" instead of "zh_CN" as the HTML lang tag for simplified Chinese pages

[cheekyshibe]

Is your feature request related to a problem? Please describe.
In Sphinx language configuration: Simplified Chinese is presented as "zh_CN". But it is not the standard HTML tag (see BCP 47 - Tags for Identifying Languages). So the auto-generated HTML lang tag will not be helpful for search engines.

sphinx/sphinx/themes/basic/layout.html

Line 107 in b8789b4

<html{% if not html5_doctype %} xmlns="http://www.w3.org/1999/xhtml"{% endif %}{% if language is not none %} lang="{{ language }}"{% endif %}>

Describe the solution you'd like
When the language is set to "zh_CN" or "zh-CN", the HTML lang tag should be "zh-CN".

Describe alternatives you've considered
Only support "zh_CN" in language configuration, but it would be treated as "zh-CN" while generating HTML lang tags.

[wendad]

Hello, I can't search Chinese in the search box on my computer, but I can search English。
I can search Chinese when I deploy it in ReadThedocs, do you know why ？

[tk0miya]

Thank you for reporting. I just posted #10311 to fix this bug.

@wendad I'm not sure. Please ask to the RtD team. We don't know at all how it works.

FYI: I'm not good at Chinese search. But it seems the Chinese search module requires "jieba" library. How about install it?

[wendad]

收到，谢谢