Is it Catholic, Protestant, or non-Christian wedding?
Is it happening in the capital or in the outskirts?
Are you caucasian? (some people might go out of their way to make you feel welcome)
Are you of African descent? (other guests might realize that you're not one of them by the way you speak), then some may treat you as Haitian, some may treat you as a foreigner.
In any case, as a guest, you should expect to have loads of fun - Eat plenty of tasty (maybe spicy foods) - meet lots of happy smiley faces.
The wedding ceremony might be a little lengthy, toasts will be more than minutes long, drving to the church and/or reception might be an 'adventure' (smoke, car fumes, and avoiding hitting a pedestrian). Most everyone will be in their sunday best.
Some reception halls can be crowded, but good food, great music under a tropical sky make for an enjoyable ambience.
Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words