Why was the Quran (or Koran) written only in Arabic?

Actually, this is a good argument against some of the claimed virtues of god, and it's simplistic and not constructive to say "this is not one of them". Making unsubstantiated claims is the pervue of the faithful.

This question goes to the heart of the nature of the god of the Quran.

1. If the god of the Quran is benevolent, then why require the people of the world to learn one particular language, or go to hell?

2. If god's desire is really to save souls, then surely publication in all languages would be a more productive use of divine influence than publishing in one language... and then hoping that people illiterate in arabic hear about the book, and are motivated enough to learn arabic.

3. If god is omnipitent and omnicient it would be trivial to produce books in all languages.

So, in answer, producing humans who natrually create many languages, but then creating Your holy book in only one language, and only saving those who read that book and are swayed enough to convert seems either silly, or perverse.

Which god? Why do you think there is anything like a god running things and writing books? If a god has never been able to appear before real human beings, don't you think it would have a hard time writing ANYTHING? In ANY language?

Why was the OT written only in Hebrew (Aramaic actually for Daniel)? Why was the NT written only in demotic Greek?

There are many excellent arguments against the existence of a biblical or Koranic god, but this is not one of them.

To avoid corruption through incorrect translations

Allah speaks only arabic. So you learn arabic or go to hell.