Fan editing help request: How can I change a movie's alien/monster/foreign language subtitles?

Hello there OT.com,

Over the last few months, I’ve been working on my first fan edit, which is a single-film cut of The Hobbit Trilogy (another to add to the pile). Along the way, I’ve been teaching myself to use DaVinci Resolve (the free version), as I have zero prior editing experience. Jumping in the deep end, I know.

So far, I’ve been able to pull off everything I want to pull off, but there’s one change I’ve wanted to be able to make that so far, I haven’t known how to do: How can I change the Black Speech subtitles?

I’ve seen other fan edits, such as the excellent Maple Edit, that change these kind of embedded subtitles, and it’s something I’d love to be able to do for my own edit (in fact it’s something I really need for a few different plot points to make sense…). I don’t know if this is actually really easy to do and I’m just missing something obvious, but any help would be greatly appreciated!