How can we reconcile the Quran's description of itself as an "Arabic Quran" with the presence of words considered colloquial within it, such as "sawwā" in "Who created and proportioned" and "jābū" in "And Thamud, who hewed rocks in the valley"?

The Holy Qur’an was revealed in a clear Arabic tongue, and understanding it requires a correct understanding of the Arabic language. The meaning of His Almighty’s saying: "Who created and proportioned" means: Who created things and proportioned and perfected their creation. And His saying: "And carved out the rocks in the valley" means: They cut and carved the rocks to make them homes and dwellings in the valley of Al-Qura.