Were they bilingual in an effort to hold on to their culture? Did new slaves coming into plantations ensure at least a few slaves still knew the old languages?
Follow-up: how likely would an enslaved person just crossing the Atlantic be to find someone who spoke their native language? Were slaves generally taken in large groups from a few of the same areas? (I know many were kidnapped and "obtained" by Europeans from certain local kingdoms, but not what proportion these represented and if the victims were arbitrary or drawn from the same rivals over time).
They developed creoles instead of properly learning english/french/spanish. A creole is, simply put, a language derived from contact between the slaves’ native languages and english/french/spanish in the context of slavery in plantations. Strangely, the imported slaves in Brazil never developed a creole language. Linguists still don’t understand why a portuguese based creole didn’t develop there, as the conditions were much the same as elsewhere.
A lot of creoles are still spoken to this day, for example in Haiti, where they speak a french based creole as their primary language.
I answered a very similar question a short while ago.