From the example you mentioned above:
That is based on the browser sending a HTTP Request header called “Accept-Language” together with the request for the website, depending on your browser’s configuration for what you have selected as your preferred language to show websites in.
In a configuration of, for example Mozilla Firefox, - (about:preferences#general):
→ Browser language: English (GB), e.g. the “en-GB” locale.
→ System language: Dansk (Danmark), … or, translated:. Danish (Denmark), e.g. the “da-DK” locale.

→ Tell websites I want to see them in the language “Chinese [zh]”, if available.
Means that the browser is sending an the above “Accept-Language” header, with it’s request, asking for the language “Chinese [zh]”, which you’re already using in your own example.
Accessing the browser language English (GB), or “en-GB” locale might be possible using client side JavaScript (e.g. within your HTML), but as that information is never sent through the HTTP request, you won’t be able to use it in e.g. redirection rules.
Accessing the system language, in the above image indicated to be “Dansk (Danmark)”, is to my knowledge not possible using client side JavaScript.
I believe Internet Explorer is the only browser I’m aware of, that has actually been exposing the operating system’s language (e.g. “da” for Danish), which was under the JavaScript variable “navigator.systemLanguage
”, but that one wouldn’t be sent with the HTTP Request, and as such, won’t be available in redirect rules either.
Maybe you’re confusing JavaScript (client-side, inside browser) with Node.js (server-side, outside browser).