How to declare language in Gatsby

For multi-language web page, it's important to tell browser and search engineer what language is for the web page content.

HTML uses lang attribute(lang - MDN Web Docs) to define language.

<html lang="zh-CN"></html>

The value of lang is defined at RFC 5646(aka BCP 47)

To modify html attribute, we need the React Helmet plugin.

    lang: 'zh-CN',