Ok, you can call window.setSubtitleLang(“ru”) and window.setAudioLang(“ru”). Just, be careful not to call them in a loop or something (too often), every change might be making new requests to Netflix’s servers and they might not like it. 
You feed it google translate style language codes (“ru”, “es”). The function returns false if it can’t find a suitable track. It will try to set regular subs first, then CC… and regular audio first, then ‘for the blind’ audio second. Huh, ok, you might want this in the other order.
Maybe this is good enough: Force Original Tracks (now a setting) ? Shouldn’t be studying instead of writing JS?