John,
I don't know if there are any official rain stats for Ambergris Caye, but Belize City gets about 8 inches in August, on average, and I would think usually San Pedro gets a little less than that, as it's farther north. The times I've been on the island in August the weather has always been nice, sunny mostly with short periods of rain ... of course, with sub-tropical weather there are no guarantees.
Most Sanpedranos are at least bi-lingual (Spanish-English), speaking Spanish with friends and family and English with visitors from the U.S. and Canada.
--Lan Sluder