Skiathos is a generally safe destination.
The summer sun tends to get very strong. You should not take the risk of dehydration, sunburns or even heat strokes lightly. Wear appropriate clothes, maybe with a hat and sunglasses; use sunscreen; bring enough water on excursions and know when to stop exhausting yourself and seek the shade of a tree.
talk
Greek is the native language of the people of Skiathos. However, most people also speak passable English and many speak German or other European languages.