Acacia is a genus, with species in both Africa and Australia: http://en.wikipedia.org/wiki/Acacia.
The German Acacia Wikipedia entry only contains the genus Acacia, while "Akazien" points to the tribe that is including all those plants that are normally known under the common name of acacia.
Btw, the picture of the English wikipedia also shows that the genus Acacia is only native to Australia. I guess they are planting Senegalia and not Acacia there. Same plant, it just got renamed 10 years ago.