Welcome Frank!
I believe we had a thread on this subject not long ago. I think we speculated that, as a Mickey Mouse nightclub operator, whoremaster and general Vegas lounge lizard, Fredo had to have visited Havana earlier--just as did millions of American tourists with less-prurient interests before Castro took over. In fact, Fredo said, "Havana's my kind of town," indicating that he had been there before.
But, I think the issue in that case was not that Fredo denied being in Havana previously, but that he denied knowing Roth and Ola--which was the source of Michael's quizzical look when he mentioned their names. Some of us here believe Michael was testing Fredo at that point.
As for the banana daquiri: Of course he should have known how to pronounce it, whether or not he'd been in Havana previously. IMO, it was another example of FFC making Fredo out to be terminally stupid, rather than an attempt by Fredo to mislead Michael.