Definition of Spanish 1: the Romance language of the largest part of
Spain and of the countries colonized by Spaniards
We see from the above definition that "Spanish" means "the language of Spain etc."
So, in this meaning, if you wrote "the Spanish" you would effectively be writing "the the language of Spain"
So, if I say, "I speak Spanish" I mean "I speak the language", if I said "I speak the Spanish" I would mean, "I speak the the language."
In the case of a translated text, we are not translating the entire Spanish language that would require us to translate a dictionary. We are instead referring to the original text. The phrase "Translated from the Spanish" is conventionally understood to mean, "Translated from the Spanish text."