May be dumb to say, but have you checked to confirm you don't have subtitles enabled? If you bring up the player controls you notice a little "cc" in the lower right hand corner (at least in the English version as shown on my TV). If you click on the cc you can configure the closed captions, turning them off or on, changing language, or changing color.
Prime has burnt in subtitles for approximately 90 % of the content outside the US. Which actually might have something to do with their garbage-tier video quality: unnecessary recodes and burnt in subtitles = multiple copies, so more incentive to use bit rates straight from the shitter.
The terrible quality and slowness I think is a remnant from them buying LoveFilm and rebranding it as Prime video all those years back, which was based on that awful Microsoft DRM that I can't remember the name of, was it Silverlight?
