I will try to answer the best I can with many findings, yet I have to add that one of the authors of a source I consulted commented that the study of Afghanistan under the beginning of the Umayyad period has been difficult due to security reasons regarding the region nowadays, so the access to sources is very limited (Azad, 2017, p. 42). This could explain the lack of information.

To begin we have to take in account that Islam was first introduced to the region through the conquest of the Umayyad caliphs at the beginning of the 700th century. "However, followers of Buddhist, Hindu, and local cults remained the majority population of many regions of Afghanistan until the late ninth century at least, when the Muslim Sa arid rulers of Sistan seized control of Buddhist Bamiyan and Hindu-Shahi Kabul" (Azad, 2017, p. 39), meaning that during that time other languages were also spoken, but taken over with Islamization thanks to Umayyad rule.

The languages that could have been spoken or starting to have an hegemony in the region due to Islamization would have been: Bactrian – the local and apparently an Iranian language written in Greek alphabet – (Malandra, 1998, p. 507) and Arabic; at least those are the languages found in some documents around that era (Azad, 2017, p. 43). Following these documents, the author explains that in Bactria the population was so diverse that people spoke Bactrian, Turkic, Syriac, Pali and Sanskrit, and notes that the Bactrian language belongs to the Persian language tree (Azad, 2017, p. 46). To quickly sum this chapter, the Umayyad dynasty brought a new religion into the region through Islamization, therefore we can imply that people were reading the Quran and the appliance of the Sharia Law, all in Arabic.

Regarding literature, I would start searching literature during the Umayyad dynasty, around the year 661-750. For that I found a document that could interest you from Suzanne Pinckney Stetkevych, which talks about the poetics during the Umayyad reign. I know the dynasty is a big one but it reached Afghanistan during the epoch you mentioned. Before that, many texts in Sanskrit were found due to the Hindu heritage. This is a speculation but you can check the book Kitab al-Afghani, maybe it has something to do with the Afghan region and the time you are asking for as it was written during the Umayyad dynasty (Stetkevych, 1997, p. 95).

Side note: you can also search for the history of the Dari language in Afghanistan, apparently it has been spoken since ancient times.

Azad, A. (2017). Afghanistan's Islam: From Conversion to the Taliban. USA: University of California Press, pp. 41-55.

Malandra, W. (1998). The Pre-Islamic Languages. Published by Taylor & Francis, Ltd: Iranian Studies, vol. 31, no. 3/4, pp. 503-515.

Stetkevych, S. P. (Jul - 1997). Umayyad Panegyric and the Poetics of Islamic Hegemony: al-Akhṭal's "Khaffa al-Qaṭīnu". Published by Brill: Journal of Arabic Literature, vol. 28, no. 2, pp. 89-122.

I hope this helps!

Edit: language