Each language should have its own folder in _i18n/, named using the ISO 639-1 language code, followed by - and the country code where relevant. A list of common ones can be found here.
Permalinks should be generated automatically from the filename, but need to be set manually for index.md pages. The lang variable should be set on all pages. Both of these, along with title (used in <title> in the head, and other metadata) are exemplified in the front matter (put at the top of the file) here:
---
title: "QMK固件"
permalink: /zh-cn/
lang: "zh-cn"
---
A summary.md is needed to render the sidebar content - _includes/en_sidebar.md should be used as the template. Links should be relativised with the language preceding it: /zh-cn/support.
The _data/strings.yml file also needs to have an entry for the language:
zh-cn: &DEFAULT_ZH_CN
<<: *DEFAULT_EN
language_name: "中文"
language: "语言"
qmk_firmware: "QMK固件"
The <<: *DEFAULT_EN allows zh-cn to inherit all values from the English entry, and the keys following it override those. It's recommended to translate all entries for full coverage.