The game runs on video files. One way of adding subtitles is to modify these video files to add subtitles over them.
I believe there is also an non-destructive way to do so by overlaying a layer subtitle using the game engine, though I’m not very knowledgeable on that. I believe @ZeroScripts had done that for another game before.
To get the subtitles, whisperAI may be handy in this age.