The Dutch word for boulder is 'kei'. However, in some instances the word 'boulder' is still used. Take for example the sport bouldering. That is referred to as 'boulderen'. The language switch might be simple in this case, but the sport itself isn't. Bouldering is a form of free climbing. People climb walls without any gear. It can be dangerous and takes a lot of agility, and core and hand strength. Once upon a time bouldering was only used as a training for mountain climbers, now it's a very popular sport. Have you ever reached the summit?