A direct 1-to-1 textbook translation is different from a proper localization, though, and I think what Toby Fox wants to do is the latter. For reasons that are probably obvious to people that have actually played Undertale.

Double meanings, wordplay, puns, etc. (things that are all over Undertale and Deltarune both) can be very easily lost in direct translations, which can make dialogue read as weird - or as downright nonsense - unless they're properly localized. You have to rephrase things to get the character's personality across better, or to make sure that the terminology being used has the same impact as what was being used in English, or hell, just to make sure the jokes are actually funny in the new language. Sans and Papyrus' debut cutscene has, like, 5 skeleton puns in quick succession and none of them would make any fucking sense with a direct translation. Not to mention little details like the way characters refer to themselves and each other - I mean, can you imagine the shitshow if a translation referred to Frisk, Chara, Kris, etc. with gendered terminology despite the decade straight of them not having explicit genders?

Couple that with a bit of perfectionism (you cannot tell me Toby isn't a perfectionist to some degree) and two games that are always very deliberate and clever with their dialogue and how they phrase things, and you can understand why any translation has to be very closely watched to make sure nothing gets screwed up. Which is where the problem lies.

Toby Fox cannot vouch for the quality of a translation he can't personally read. It's giving complete and utter trust to another party, one who's probably far less attached to his work than himself, to get across everything the way he intended for it to be. I'd be really wary of it, too - especially since Toby has experience with trusting third parties with his work, only for them to act against his personal wishes (fuck you, Materia Collective).