Until World War I, the term German could have referred to peoples from areas of what is today the states Germany, Poland, Belgium, Luxembourg, Czech Republic, Slovakia, Austria, and even Croatia, and the Netherlands.
Prior to 1789 there were over 300 independent sovereign states and thousands of other bishoprics, lordships, and free cities that made up the Holy Roman Empire.
[1] When the term German Prince or Princess is used, it is referring to a member of one of the many ruling sovereign houses of the region of Central Europe.
They were often used as suitable marriage candidates for royalty, simply due to the fact there were so many of them.
Also, as all of the European royalty were related, it helped to use a German prince/princess to keep the marriage "in the family", so to speak, or sometimes the opposite: to broaden the gene pool.