How to change the html attribute lang?

[Hkr32]

By default it is equal to lang="en"
I need to replace with lang="ru"

[slorber]

You can use the i18n config even if you don't plan to translate your site:

https://docusaurus.io/docs/docusaurus.config.js#i18n

module.exports = {
  i18n: {
    defaultLocale: 'ru',
  },
};

This will also automatically translate theme labels to Russian, using these translations (that you can override):
https://github.com/facebook/docusaurus/blob/master/packages/docusaurus-theme-classic/codeTranslations/ru.json