Question: In general in English, we don't ever apply the definitive article to languages. We don't say "He speaks the Japanese" or "It was originally written in the French."

But for translated books, they are very often prefaced with a note phrased as *Translated from the Spanish* or *Translated from the Arabic*.

Where does this odd form originate? What is the reason for this grammatical deviation?

Answer: 
“the *adj*” is a reduced form that removes a noun (which is usually obvious from context) because the adjective is what really matters.

In this case, “the Spanish” probably means “the Spanish version”, though there are several other words that would give the same overall meaning.
> 
> Definition of Spanish 1: the Romance language of the largest part of
> Spain and of the countries colonized by Spaniards
> <https://www.merriam-webster.com/dictionary/Spanish>
> 
> 
> 

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."