I think it just defaults to the user's OS language (iOS, browser). Fortunately once you turn it off (or set it to another language) - that setting will persist

I think it just defaults to the user's OS language (iOS, browser). Fortunately once you turn it off (or set it to another language) - that setting will persist so you don't have to constantly change it back. I flip pretty frequently between Chinese and English subs and it'll remember the last setting between episodes / shows / etc.