Whatever Spanish you know, use it. The sun's ultraviolet rays are much more direct and stronger in Costa Rica because it is only 10 degrees north of the equator. Many sites you may visit are at high altitudes which means you'll burn even more. You may not feel your being burned until it's too late. You can also be burned in overcast conditions.
Some people prefer long sleeves and long pants.
They both work. If you're a person who doesn't burn in the temperate zone and want to know what a sunburn feels like, don't follow this advice. Sunglasses are also recommended. Although very few tourists have been bitten by poisonous snakes in Costa Rica, they do exist in the areas to which you will be traveling. Without becoming paranoid, a few precautions are advisable.