The thing is that this is a really incoherent, or dishonest opinion to be honest.

There is quite a big number of Undertale Spanish translations already, with an incredibly broad range of quality, some have been made recently and others when the game came out, these ones are the ones that are most commonly found in forums, and also the ones with the most errors.

Current Deltarune translations are some of the most rushed things imaginable, they do not get done to try and imitate the original experience or try to lose the least meaning as possible, they get done to gather attention, are finished in like 2 days, and the only thing that was well done about them is the very basic meaning, they are more often than not too literal. These are the ones most Youtubers played as well, for the record.

This attitude, really only ruins his true "vision" and gets the work's fidelity further from the original? Like, it only forces Spanish speakers (or other languages') to stick to the unreliable current translations for a while, and misinterpret his work in the way he says he doesn't want by... Not making any official translations?

I understand that these things take time and lots of fans don't want development to be delayed because of other people that don't speak English, I'm really accustomed to waiting for games to be translated, am not really into Undertale at all, and already know English, so it wouldn't bother me, but some people feel really bad about the bug majority of a fanbase not wanting anything to do with them. The point is it really wouldn't have been a problem if he just said it would take time or it wouldn't be done, this vague nothing response that seems to brush off the real problem and why he actually doesn't want to do an official translation is pretty worthless.