Regarding enslaved & free African Americans speaking languages other than English, see this excerpt about the abolitionist speaker/activist, Sojourner Truth:

"Sojourner Truth was born in 1797 in Ulster County, a Dutch settlement in upstate New York. Her given name was Isabella Baumfree. She was one of 13 children born to slave parents. She spoke only Dutch until she was sold from her family around the age of eleven. Because of the cruel treatment she suffered at the hands of her new master she learned to speak English quickly, but would continue to speak with a Dutch accent for the rest of her life"

-snip-

Here's additional information about Sojourner Truth:

She eventually added abolitionism and women's suffrage to her oratory, often giving personal testimony about her experiences as a slave. In 1851, she spoke at a women's covention in Akron, Ohio. The legendary phrase, "Ain't I a Woman?" was associated with Truth after this speech.

After the Civil War ended, she worked tirelessly to aid the newly-freed southern slaves. She even attempted to petition Congress to give the ex-slaves land in the "new West." Truth continued preaching and lecturing until ill health forced her to retire."

Source:
Sojourner Truth