Set the visitor UI language automatically

Ok, mdfw  https://forums.ni.com/?profile.language=zh-CN (for example) does work on your community, and according to everything I can find, the only way to set a language for an anonymous user is by setting this url parameter. 

So I would use the combination of tricks I mentioned above (probably starting with a cookie) to let the community know to change the language in the url for this user. As far as I can tell so far, that's your only option for anonymous users. 

So, on load just check if there's a cookie with the languageValue value, if so, something like (pseudocode, use whatever cookie code style you prefer😞

if(cookie exists with value = langaugeValue){
  window.location = yoururl + "?profile.language="+languageValue
}

if not drop a cookie for next time

set a cookie = "languageValue=XX; expires=whatever date; path=/";