I've been doing some thinking about the situation, and I'm wondering if part of the reason why Toby is apprehensive about translations into languages he doesn't personally speak is because of AI. In the indie scene, there are a lot of fan groups that will translate games (usually ones in Unity, Renpy, or any other engine where it's well-documented how exactly to decompile the game and modify it)- Russian and Chinese are the most common, but Spanish and other languages crop up from time to time too. The thing is, a lot of these groups will very heavily use AI for their translation, which is still far from perfect, especially translating things from English, in my experience. The devs I've talked to really don't like this (partially because they are morally opposed to AI,) but also because they fear the shoddy translations make their games come across as much worse, especially something very story and comedy driven like Deltarune, and they'd have no way of checking for themselves, since they can't read it. A lot of these groups don't ask for permission, but for the ones that do, the devs nearly always will stipulate that they make it absolutely clear that their translation is unofficial and not endorsed by the dev, just to distance themselves from any low quality AI-generated translations. Obviously, low quality human translations have and still do exist (I think they call those low quality translators "localizers," these days,) but AI has significantly increased the number and concentration of sloppy, bad-impression-giving translations in the indie scene.
My thoughts are that in addition to his desire to supervise the translation process to make sure his vision stays intact, he also would be concerned that if he outsourced a translation, his game might receive a crummy AI translation and he'd be none the wiser, since he couldn't read it for himself to ensure the finished translation is human-made.
Are there any statements made by Toby about translations back when Undertale came out? If his perspective changed since then, that'd definitely make it look more likely to me that the rise in AI translations have made him all the more protective of his writing