There are a lot of references to Undertale in Deltarune, I can even think of a scene that makes a bit less sense without knowing Undertale first, so you should start with Undertale.
This is the first post here by someone other than me btw, thanks for making this place a little less empty
Edit: oh right, Deltarune isn't even finished yet, so it'd also be simpler to play UT, then DR than the first 2 DR chapters, UT, and DR again.
6
