Actually, the Italian does not use “form” at all, only the German does. The Italian merely speaks of a concretization of the concept. The German turns this into “concrete form.”
It would have been better to use a verb: “so that the concept of a family of nations might be really concretized”
in both the German and the English, though this is hard to do in German—the verbs available tend to be abstractifying Latinizations. “Konkretiert” can, I suppose, be used, and perhaps it would be just odd enough to make the point.
“so that the concept of a family of nations might be solidified”
IMHO.