A Langjökull tour is perfectly safe if you’re led by a local guide. They’ll provide all of the gear you need—such as an icepick, a harness, and crampons—and show you how to use them correctly. They’ll also lead you through the safest areas of the ice cap.
That said, if you’re going alone, we can’t guarantee your safety. Glaciers are complex and hazardous environments for the inexperienced.