Why Undertale and Deltarune Haven't Been Officially Translated to Russian and Other Languages? tobyfox Responded to Criticism

Unfortunately, Undertale does not have official translations into other languages besides Japanese. There is no translation into Spanish and Brazilian Portuguese, as well as all other languages — Chinese, Korean, Russian, French [...and many others]. I understand that the lack of an official translation into your language is very unpleasant. However, I want to clarify: this is not because I am biased towards other countries.

It's just that if I release something officially, I want it to fully correspond to my vision. I was able to make a translation only into Japanese, because I know this language myself and worked closely with our talented translator, carefully checking the entire text. I am not against official translations into other languages, if we can implement them with the same level of quality… I have considered different options with my publisher before, but nothing has come of it yet. Perhaps it is worth trying a different approach.

Given that I don't understand other languages, the ability to allow fans to interpret the game themselves without pressure to achieve "official perfection" is incredibly helpful.

From a development point of view, it is simply impossible to support other languages now, because it will significantly slow down the release of the game…