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.
navigator.systemLanguage”, but that one wouldn’t be sent with the HTTP Request, and as such, won’t be available in redirect rules either.