I asked a Czech friend to render an accurate translation, and here’s what he said: "For humanity, which is struggling with the implications of wars, hatred, and massive criminality, the expansion of the EU to include Czechs, Slovaks, and other nations is only a small change." It’s definitely “nations” (národy), not “nationalities” (národnosti), meaning "nation" in the sense of "a people." I think Jews and Roma are probably národ to the Czechs, just as Isreal is "a nation" in the Old Testament even when it's just a bunch of tribes wandering around in the wilderness.
In the Czech Republic and perhaps most of Europe, nation means something other than state or country, just as "nationality" is a different category than "citizenship." I believe there's actually a space on the Czech passport for “nationality,” as though having a Czech passport doesn't by definition make you Czech.
All this sounds really bizarre and old-fashioned and vaguely creepy to me, which is another way of saying that deep down inside, my narod must be American. And so the title of this post should be nous sommes tous Américains. (OK, and Canadians and Australians.)
We ended up changing it to “people” because to the average American reader, it otherwise wouldn’t make much sense.