He represents Germany, no article. Countries normally take no articles. That would be used, say, at the United Nations.
He represents a civilized Germany, with article.
It is not really defined. It is like: He is a new breed of entrepreneur. Same idea. Undefined. General.
It is not: the Germany of World War I or the barbarous Germany of 1942.
And: He represents civilized Germany. would imply there is an uncivilized one, too. Or that civilized Germany is being compared to uncivilized Germany or some other adjective plus country name.
He represents "nice" Manhattan as opposed to "nasty" Brooklyn.
Same idea: No determiner suggests a contrast to another adjective, same place, or another adjective and another place.