Joerg, it sounds like the Wanderngitarre might be unique to Germany. From the use you described, it could indeed be called a backpacking guitar, but it sounds like the particular design of it is different from the other backpacking guitars I've seen. Over here, the term is used for any number of different instruments that are made small enough to be carried rather easily. There are many different styles. Some are fairly cheap, others are fairly expensive. They are usually made to be played like a regular guitar, but are designed more for portability than ruggedness. Martin makes one that has a very narrow trapezoidal body, flared out to perhaps 10" (25cm) at the wide end (good tone but very little volume, especially in the bass). Taylor and Larrivee (sp?) make "baby" guitars that are like 2/3 scale models of their full-size instruments.

Perhaps the most accurate term in English would be "German backpacking guitar".