There appears to be very little crime on Formentera especially compared to Ibiza. You do not have to worry about leaving your bag on the beach when you take a swim, or worry about leaving your moped out on the street.
Be careful with walking on the asphalt road in the dark. The island is notorious for people often tourists drinking and driving. In general the car driving style on the island is not very subtle.
talk
On Formentera the Catalan dialect formenterenq is the main language, but Standard Spanish is spoken by everyone as well.