An Arabic Qur'an...
Allah tells us continuously that the Qur'an is in the Arabic language, a response to those who might argue that translations in other languages can be a replacement.
So we notice that whenever Allah mentions the Qur'an in a language, it is none other than Arabic;
إِنَّا أَنزَلْنَاهُ قُرْآنًا عَرَبِيًّا لَّعَلَّكُمْ تَعْقِلُونَ
Indeed, We have sent it down as an Arabic Qur'an [Qur'anan 'arabiyyan] that you might understand.
وَكَذَٰلِكَ أَنزَلْنَاهُ حُكْمًا عَرَبِيًّا
And thus have We sent it (the Quran) down to be a judgement of authority in Arabic.. [hukman arabiyyan].
وَكَذَٰلِكَ أَنزَلْنَاهُ قُرْآنًا عَرَبِيًّا
And thus We have sent it down as an Arabic Qur'an [qur'anan 'arabiyyan].. [Qur'an Taha 20:113]
Also see the ayaat/verses in Surahs to prove this further;
* (Yusuf 12:2), (ar-Ra'd 13:37), (Taha - 20:113) (Zumar 39:28), (Fussilat 41:3), (ash-Shuraa 42:7) (az-Zukhruf 43:3), (al Ahqaf 46:12)