I’m in Japan, with an English-language Amazon account, yet Prime insists on displaying Japanese subtitles on absolutely every thing I watch. Doesn’t matter what the original language of the content is, doesn’t provide an option to turn it off. Huge, bright white subtitles - much bigger than what Netflix uses. Been this way for years.
Sometimes I have fantasies about sending an email direct to Jeff Bezos just to say: dude, did you know about this?
Suffice to say, I don’t watch much Prime.