Having established the divine origin of language to his own satisfaction, Ibn Hazm goes on to discuss the vexed question of what language Adam spoke, and concludes - sensibly enough - that there is no way to be certain. However, he figures it must have been "the most complete of all languages, and the most clearly expressive, and the least ambiguous, and the most concise, and the one with the most various names for all various nameable things in the world" since, having been bestowed by God directly, it must naturally have been the most perfect.
He also decides that it almost surely must have been the ancestor of all modern languages, because it was not inconceivable but extremely improbable that anyone would have decided to waste so much time and effort as to "invent a new language, which would be an enormous effort for no reason; such meddling would not be undertaken by any intelligent person... [its inventor would be] a person busying himself with what does not benefit him and neglecting what concerns him", and even if this did happen, it was even less likely that the inventor would be in a position to impose his language on any community. He specifically considers the "Esperanto" case of a multilingual kingdom adopting a common lingua franca, and argues that "it would be easier for him [the king] to make them learn one of those languages that they used to speak, or his own language; this would be easier and more plausible than the invention of a new language afresh."
He concludes by tersely stating that "Some people imagine that their own language is the best of all languages; this is meaningless" and justifying it theologically and logically.
I like this guy. Makes me wonder what other early linguists had to say...