This one’s tough. Jicama is also called Mexican potato, but that’s not going to help you much. It is a large, root vegetable with a thin brown skin and a white crunchy flesh. It is peeled and eaten raw or cooked. In texture, and to a lesser extent in taste, it is very similar to the water chestnut.
It is becoming more and more popular in Asia and the US, but hasn’t been widely used outside Mexico until recently. We have no idea where or if you can track it down in Germany. If you’re using it uncooked in a salad, the best substitution would probably be a firm, not-too-sweet apple. If you’re using it in a cooked dish, we’d go the water-chestnut route.