Obviously, they would not have all spoken the same languages, but is there any study on what languages were most common? Would it mostly have been languages spoken in West Africa? And for that matter, why did those languages die out? Were there direct policies of linguistic oppression against slaves or was it due to there being too many languages and no incentive to keep them?
The slaves imported from Africa came from a huge range of African people. Some of these Africans already had contact with European traders and would have some who understood the language of European traders, especially on the coast of West Africa.
According to the source left at the end of the comment, Niger-congo languages (1400+ distinct languages) would've been the most populated language known by the slaves brought to America for the triangle trade participators. Due to the mix of languages and the need to understand European languages there was an evolution of african/euro languages to pidgin and creole languages.
https://www.nps.gov/ethnography/aah/aaheritage/sysMeaning_furthRdg1.htm