nationality openings

Often it's based on a player or group of players from that nation having played the opening a lot and/or having developed its theory. For example, the Greco Counter-Gambit ended up re-named as the Latvian Gambit last century because a group of Latvian Masters studied and played it. If I recall aright, I think the French Defence got its name because it was played by Paris in a nineteenth-century London-Paris correspondence chess consultation game. And so on.

Actually, I can tell you how the French Defense got its name. In a team match in 1834 between some area of Britain and France, the French team played 1...e6, to which their opponents played the Exchange Variation and barely made it to move 30. There is actually question as to whether White resigned move 29 or 31. After this 1834 team match, 1.e4 e6 2.d4 d5 was dubbed the French Defense.