Yooo guys, I’m getting into speedrunning Deltarune, but idk if I can use Japanese as the games language if I don’t speak it. Is that banned? It’s not in game rules. i’m grateful for any and all help.

You can use Japanese, and it's actually preferred because it saves about 10 seconds due to Japanese sentences being shorter.
Many runners, (including me), speedrun in Japanese because of this reason.

The language doesn’t matter as you’re just trying to beat the game as fast as possible. That should go for all speed games.

What we don’t allow is languages outside English and Japanese as that requires you using an unofficial version of the game but since Japanese has been implemented into the game, we allow it.

it doesnt matter what language you use
i just use english because then i can understand what option i use
but Japanese saves some time