Linguistically the concept of foreign words in languages is described as ‘borrowing’ . The idea of borrowing words is a universal observation that applies to all languages. English is a good example of this; take the word ‘Philosophy’. It comes from the Greek words ‘Philo’ which means love of and ‘Sophia’ which means wisdom. The concept of ‘foreign’ or ‘borrowed’ words comes from the fact that different races, cultures and peoples come in contact with one another .
What was specific about the Arabs at the time of revelation was that they came into contact with other cultures due to trade and had subsequently borrowed certain words. These foreign words in the Qur’an had already been naturalised into the Arabic language before the revelation of the Qur’an . These words were already in use in the Arabic language.  According to Imam Shafi’i  these words had been fully integrated into Arabic and were already a part of the language .
These foreign words include:
Mount (Qur’an 95:2) borrowed from Syriac
Heavy (Qur’an 18:31) from Persian
Sinai (Qur’an 95:2) from Nabatean
The Inscription (Qur’an 18:9) from Greek
The Sea (Qur’an 7:136) from Coptic
Brilliant (Qur’an 24:35) from Abyssinian
To turn onto someone (Qur’an 7:156) from Hebrew
With reference to the Qur’anic statement that it is a “Plain Arabic Qur’an”  al-Suyuti believes that the presence of a few foreign words does not make it any less Arabic then the presence of foreign words in a Persian Poem would not make it any less Persian . Additionally the reference to “Plain Arabic” is to the Qur’an as a whole, and not the individual words in it .
To conclude, claims made by some critics are debased by understanding the nature of languages and how they naturalise foreign words into their vocabulary. This phenomenon happens as a result of different cultures and races integrating and coming together. Furthermore the Qur’an can be described as ‘plain Arabic’ because the foreign words in the Qur’an had already been naturalised and were already part of the Arabic language before revelation.
 G. Yule. 1985. The Study of Language. Cambridge University Press, p. 52
 H. Abul-Raof. 2003. Exploring the Qur’an. Al-Makhtoum Institute Academic Press, p. 38
 F. E. Peters. 1994. Muhammad and the Origins of Islam. State University of New York, Chapter 3; and Hasan Dhiya’ al-Din. 1988. al-Ahruf al-Sab’ah wa-Manzilat al-Qira’at Minha. Beirut: Dar al-Basha’ir al-Islamiyyah, p. 37
 Exploring the Qur’an, p. 39
 Died in 820 CE
 Cited in A. Jeffrey. 1938. The Foreign Vocabulary of the Qur’an. Baroda: Oriental Institute, p. 110
 Exploring the Qur’an, p. 39-40
 Qur’an Chapter 16 Verse 103
 al-Suyuti. 1996. al-Itqan fi Ulum al-Qur’an. Vol. 1. Beirut: Dar Ihya’ al-Ulum, p. 367
 Exploring the Qur’an, p. 40