How do we respond to those who challenge the Qur'an, claiming it contains foreign words despite being "Arabic and clear"?

This statement does not deserve a response because it is obstinacy and sophistry. The Holy Quran is the pinnacle of the Arabic language and its highest degree, as testified by the Arabs of the era in which it was revealed, who were masters of eloquence. And the truth is what even enemies testify to. As for the words found within it that were spoken by non-Arabs, this is due to the concurrence of languages.