for a people who know
لقوم يعلمون
They understand it, and they are the Arabs. This means that Allah made it an Arabic Qur'an for a people who know it and understand it, and they have no excuse for opposing it and disbelieving in it, because they know it. As He the Exalted said in another verse:
Indeed, We have made it an Arabic Qur'an that you might understand.
إنا جعلناه قرآنا عربيا لعلكم تعقلون [Az-Zukhruf 43:3]
meaning that you understand its meaning, since it came in the language of the Arabs, ....