Language code might differ from RSS Spec

[YDX-2147483647]

Example

• Mkdocs: en_US. (underscore)
• RSS Spec: en-US. (hyphen)

Note: RSS also accepted lowercase en-us.

Explaination

• theme.locale of Mkdocs basically uses ISO-639-1 2-letter abbreviation (en), and may also include a territory suffix (_US).
• And the language code in RSS follows W3C / RFC 1766 / full ISO-639. I guess it's consistent with HTML, which follows RFC 5646 (a newer version of RFC 1766).

Additionally, you can't write hyphen (en-US) in mkdocs.yml.

$ mkdocs build
ERROR    -  Config value 'theme': Invalid value for locale: expected only letters, got 'en-US'
Aborted with 1 Configuration Errors!

Possible fix

(I can turn it into a PR if you want)

Change the following line in .util.Util.guess_locale

mkdocs-rss-plugin/mkdocs_rss_plugin/util.py

Line 563 in 769744e

return mkdocs_config.get("theme")._vars.get("locale")

to

locale = mkdocs_config.get("theme")._vars.get("locale")
return f"{locale.language}-{locale.territory}"

[Guts]

Nice catch!

(I can turn it into a PR if you want)

Please, feel free!