The short answer is: speak and write in Haitian Creole. Here’s why…
When you speak and write in Haitian Creole you communicate more accurately and clearly. All Haitians understand Haitian Creole. When you use French, one of the two official national languages in Haiti, you make Haitians translate fairly large grammatical differences on the fly.
When you speak and write in Haitian Creole you create more engagement with your audience by honoring the Haitian culture. More engagement typically means better understanding. And isn’t that always your goal?
When you speak and write in Haitian Creole you get your message across to more people. Only 5% of Haitians are fluent in French, Haitians prefer Haitian Creole (studies show even younger Haitians with a working knowledge of French prefer to both read and write in Haitian Creole), and only 5-10% more understand “some French”.
A better version of the short answer to how to communicate effectively with Haitian people: use a translator who is a native Haitian Creole speaker. Haitian Creole has its own grammar and writing system that is newer and more modern than the older languages it derives from. Literal translations from those older languages using their grammar can lead to confusion. Sometimes humorous (All applicants must be completely filled out by the forms.) and, unfortunately, often resulting in misunderstandings. Haitian Creole only uses the active voice; it does not use the passive voice. Native Haitian Creole speakers can translate from passive to active when most cannot.