Quran's Arabic
Origin or word in some tribe or a country is a thing and its use in a language or dialect is another. Quran used the words which were Arbanized from Persian too, like Sijjeel, Siraaj. Once a word is adopted by a language it is now its own word. as Urdu has adopted the words like computer, Radio etc, no matter they have different origins. So Quran is revealed in the language of Quriash is a valid claim. The claim of Quran that it is Mubeen (clear to understand) can only be confirmed if its audience had no objection to understand its language. No Qurashid every objected that the Quran is not in its own language or dialect and thus somewhat strange or alien or unfamiliar.