It is possible to generalize about nationalistic aesthetics, despite the fact that there always has been and always will be composers who cross boundaries in compositional style, though usually there still is a hint of nationalistic tradition.
When looking at more "traditional" music, and with that I mean the bulk of the standard rep (discounting serialism, computer aided music, and the like), French music, since the Romantic era at least, has tended to have a more flambouyant character than say, music from Eastern Europe. I don't think anyone would disagree about this generalization. This goes along with, and is connected to tonal concepts that have traditionally been associated with French and French style woodwinds...a fairly bright tonal color. Talk to orchestral woodwind players about this and they will confirm it. Interestingly, most American and a majority of European orchestras employ a more "Germanic" tonal concept within the woodwinds. In American orchestras it has evolved to a concept that is equally different from French woodwinds, but different enough from German that is has to be called American.
A look at musical interpretation between French style saxophonists and Rascher influenced saxophonists will also seem to support the notion that French music generally is more characterized by a virtuosity that is closely related to finger dexterity.
Obviously there are exceptions to everything, and as generalizations can be helpful in identifying trends and such, I think it is alright to make them here.
Even after WWII, I believe there still is nationalistic compositional styles, though to a much smaller degree than in the past. Our differing cultures almost dictate that there will be differences, just because of the music, art, dialogue, etc., that surrounds us in our country of origin.
Composers from America may write music with a Spanish flavor, or Russian composers for a French ballet, but they are doing so as a compositional tool. Copland is was trying to make El Salon Mexico sound latin, instead of just writing what he heard. Stravinsky probably didn't change any of his ballet music, but simply wrote it for a French ballet. My point is they write music from within. If a composer has emigrated and has spent enough time in a new country and has really been submerged in the new culture, then their music may reflect that. It's another one of those grey areas...Rascher lived most of his life in America, but he still had a German accent to his last days and German remained his most comfortable language.