African slaves arriving in the United States came from a wide variety of groups, which often spoke different languages and couldn't understand one another. Picking up the language of their masters was the simplest, easiest, and most advantageous form of communication. While English was the language of choice, it was certainly not the same English that we know today, and would be hard to understand to unfamiliar ears. Through a different set of influences and no cultural pressure to conform to a certain way of speech, African American English developed separately from Standard American English, and the modern day African American Vernacular English you see today in African American neighborhoods is a modern, much more assimilated descendant of this slave dialect. In some small, remote parts of the United States, some communities speak a language called Gullah, which is a more isolated, diverged form of the dialect distinct enough to be considered a separate language.

You probably asked that for north america, but i can tell you about in the south america. They are similar but different in some aspects. First of all, like the other poster said, the african slaves were captured in the indigenous conflicts inside africa and sold to the portuguese outposts (mainly in mocambique and angola, but not only there) and slave traders, so they are from different linguistic and social groups, often with their own quarrels. The slave traders usually separated people of the same group and sold them separatedly, if possible. That doesnt mean that didnt happen to have large groups of the same ethinicity. For example, the malês rebellion (1835) in Salvador was made by islamized africans of the nagõ etnicity. I dont recall seeing that ethinicity in the documents of the south of Brazil, nor reading anything about islamized slaves outside rio de janeiro and salvador. But that doesnt mean that those people were passive to the slavery. Even if they were forbidden by the church of speaking their original language and cultuated their gods, they couldnt leave their own unconscious identity (it does have a better formal term, but i cant recall how it is in english). Also, some groups of fugitive slaves formed the "quilombolas communities", something like a haven for fugitive slaves and indians, where they tried to recreate some of their culture and rituals (like electing a king and shamanical practices). Some of those communities still exist today, but they dont have an original language, but use portuguese as a base of communication while they use their own african names for most things. They usually have a nice dialogue with their african counterparts.