Add support for `ISO 639-2` and `ISO 639-3` codes

[figure002]

The following language is rejected by mkdocs-static-i18n:

plugins:
  - i18n:
      languages:
        - locale: ast
          name: "Asturianu"
          build: true

It responds with an error:

ERROR - Config value 'plugins': Plugin 'i18n' option 'languages': Sub-option 'locale': Language code values must be either ISO-639-1 lower case or represented with their territory/region/country codes, received 'ast' expected forms examples: 'en' or 'en-US' or 'en_US'.

But there exists no ISO 639-1 code for Asturian; there are only ISO 639-2 and ISO 639-3 codes, see https://en.wikipedia.org/wiki/Asturian_language.

Would it be possible to add support for ISO 639-2 and ISO 639-3 codes?

[figure002]

I'm willing to work on a pull request, but I read that "mkdocs-material only supports ISO-639-1 country codes" (1). Is this still the case? I use mkdocs-material, but seeing that mkdocs-material is no longer receiving features, fixing this might not be trivial.

[ultrabug]

Indeed @figure002 mkdocs-material is officially frozen while mkdocs itself is also blocked by maintainership confusion...

This is part of why this repo is not receiving much attention from my end as the future of mkdocs is not clear.

That being said, before you try to use this plugin did you try having a simple mkdocs repo and use ast with mkdocs-material? What does it do/say?

[figure002]

I assume you mean this syntax:

extra:
  alternate:
    - name: English
      link: /en/ 
      lang: en
    - name: Deutsch
      link: /de/
      lang: de

I tried this, but going down that route is a huge step backward from the convenience that mkdocs-static-i18n provides, especially since my documentation is translated into over 40 languages.

If I were to fix it in mkdocs-static-i18n, would that mean it still wouldn't work due to mkdocs-material's limitation?

[figure002]

@ultrabug I also did an integration test and it works as expected. I'm able to build a language with 3-letter language codes.