I finally found the subtitles hidden in a png file.
The file content-type was "plain/text" instead of "image/png".
I should have noticed it sooner
I then found how to decrypt and decode the file through a function in a swf file loaded as an addon with the player.
I now got the subtitles in a .ass format
So now i'll just have to automate it through a script to get the videos + subtitles and then mux them