And We indeed know that they say, “”Only a man teaches him.”” The language of the person towards whom they incline (doubting that he teaches the messenger of Allah) is foreign and this is clear Arabic language.
The infidels had raised all sorts of doubts about the Divine origin of the Quran, one of these doubts was that some person teaches him the Quran. It is clarified that the language of the person they suspect is not Arabic then how can he teach in Arabic that too so clear.