In this case it's not too bad, but I think it's probably better to translate repeated sentence endings like the "na" here. It just feels weird to have a Japanese grammatical particle mixed in with otherwise English text. Stuff like "It's a demon realm plant, ain't it?" and "I must've surprised you, ain't I?!" would feel more natural while still retaining the "repetitive weird thing vibe," I think. (It doesn't have to be "ain't" specifically, though. You can do whatever you think fits best.) You also wouldn't have to explain what it was in a translator's note.