I lived there for 8 years, but not on the Pacific side. There are parts of the country (higher elevations) that can get quite cold. Where we were, it would be very hot and humid during the day but very cold at night (to the point where we'd have to wear winter pajamas and use blankets). If you visit the volcanos, you will most likely need a jacket. Hope this helps.
I've also heard that Costa Rica is not as safe as it used to be. Don't wear nice jewelry, and pretend you're in the U.S. The problems I've heard about have to do with drugs. Theft has always been a problem there, which is why so many people have wrought iron gates around their properties.
Lastly, English is not spoken everywhere. Where we lived, hardly anyone spoke English.