You may know what an interesting and challenging resource Undertale game is, but there's a new chapter to the story, and it was released simultaneously in English and Japanese.
The language in Deltarune itself is quite challenging. Dialogue is heavily stylized based on the speaker, there's slang, weird jokes, and strange spelling and character use. This isn't for the faint of heart, but if you made it through the Japanese version of Undertale, you'll probably be fine.
If you're looking for a challenge, and if you're planning to play it anyway, download the Japanese version. Then again, keep in mind that you can't switch back and forth between languages in Deltarune like you could in Undertale, but you can always download copies of both versions. They're free, after all.