* refactor: migrate i18n files from YAML to TOML format and simplify structure
* refactor: make translation file adjust to gohugoio/locales language code so date translation can work properly
* refactor: use hyphens instead of underscores for i18n, because that's how Hugo pick the translation file
* docs: update comments for i18n language availability and CJK language detection in hugo.toml.
