Reddit mentions of Many Thousands Gone: The First Two Centuries of Slavery in North America

Slaves originated from a variety of places and were brought over by a variety of European nations. Different parts of the country imported slaves from different areas as well. Some of them would have already known English (or Dutch, French, etc) from prior plantations in the West Indies, some of them would not speak a word. I think the first slaves in New York were creoles, so they would have spoken English with no trouble. The Carolinas brought slaves mainly from Barbados, as another example.

Assuming we're talking about someone captured with no prior contact to Europeans, the explanation given by Ira Berlin in Many Thousands Gone is that it depends on the task. If you're being sent to work rice or cotton, it doesn't matter if you speak the language. You just worked and struggled on. If you're a carpenter, horsemen, driver, or other complex task you're going to be exposed to the language more and picking it up from the people around you.

Importing slaves into America was banned in 1807 because of fear about the rising population of slaves. By the time the Civil War came around they would all have been born on American soil and fluent in English.

Except it didn't take 400 years. Slaves fought back from the very beginning. You should read Many Thousands Gone if you're genuinely interested.