Why do translations refer to the original language with a definite article, e.g. "translated from the Spanish"?

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?

“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.

In English, the definite article "the" has often been used in an idiomatic way with the names of things that wouldn’t appear to need an article..

Once, the use of "the" with a language was much more prevalent than it is today. Here are two old citations from the Oxford English Dictionary:

"Let not your studying the French make you neglect the English" (1760).

"Every advantage that … a complete knowledge of the Arabic could afford" (1795).

The OED says people use "the" with languages in an elliptical way – that is, they’re mentally deleting part of a longer phrase. Examples: "translated from the Spanish [version]" … or "from the [original] German" or "from the Japanese [language]."

According to an online article:

Rule 7.12: Use the definite article when the word language immediately follows the name of a language.

English is hard.

The English language is hard.

Bill wants to learn Chinese.

Bill wants to learn the Chinese language.

It's similar to asking the question "What's the Spanish for -something-". For example "What's the Spanish for Supermarket?"

In that case someone is asking for a specific Spanish word (the answer is 'supermercado'). In the case of "Translated from the Spanish" the writer is referring to a specific Spanish text. For example if the quote related to the windmills passage in Don Quixote the English might have the subscript "Translated from the Spanish" where "The Spanish" related to that passage in Don Quixote and not to, say, a guide to the Alhambra.

When we say "Does he speak Spanish?" the question is about the subject's ability to speak (and understand) Spanish generally. This would include the ability to read Don Quixote, understand a sound guide to the Alhambra and to describe a fault with his car to a Spanish mechanic.