Zola is validating language incorrectly

[pickfire]

Bug Report

Environment

Zola version: 0.17.2

Expected Behavior

zh-CN should not be valid language and should be suggested to use zh-Hans-CN or zh-Hans instead. I have seen zh-CN quite often in quite a few sites since it renders differently if it is done incorrectly and less readable.

Current Behavior

zh-CN passed language validation check since it uses unic-langid but is not recognized by browser, it should instead use zh-Hans which is specified by https://datatracker.ietf.org/doc/html/rfc5646#appendix-A and https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang.

I filled an issue to hexo too https://github.com/hexojs/hexo/issues/5186.

Step to reproduce

1. Install a theme (I didn't expect to have to install a theme just to test this out, and the first that I installed even broke, I almost want to ignore writing this issue)
2. Set default_language = "zh-CN" in config.toml.

[Keats]

Do you know a crate that validates language code for the browser?

[pyfisch]

Both tags are valid, for a static site generator it might be useful to suggest adding a script to certain language codes.

[pickfire]

Let me investigate on this, I think I need to check what needs to be done and if needed probably a separate crate will be done.

[jonassmedegaard]

Looks like the crate language-tags does exactly what is needed here.

[jonassmedegaard]

Another crate seemingly doing exactly what is needed here is fluent-langneg, also given that zola might use Fluent in future.

[Keats]

fluent-langneg sounds like a good solution