I’ve made such a box myself. It cost me $20 in hardware and many hours of effort.

https://tomverbeure.github.io/2018/04/23/Color3-HDMI-RX-to-H...

Right now it’s just a proof of concept (check out the video with a moving overlay rectangle and the red 8 that is converted into a partially green 8), but it wouldn’t take a whole lot of work to add support for subtitles.

There are still some of these boxes for sale on Amazon, but supply is limited.