The World of Greyhawk includes a large number of nation-states. While the gazetteer does a good job of providing population figures and natural resources and details of the size and composition of the nations' armies and traditional alliances and enmities, it doesn't have much to say about their culture. Some hints can be picked up (mostly in connection with the "exotic" realms near the edges of the map), but there's tendency to just portray most of them as having the same "generic fantasy" monoculture. Gary's Greyhawk novels didn't really help with this.
Therefore, in order to add more flavor and make things a bit more interesting for myself, I've found it helps me to visualize and differentiate in my mind the various states of the Flanaess to figure out which earth-nation they seem to most closely approximate and use that as a guide towards personifying their cultural attitudes and incidental color (i.e. stuff like accents, naming conventions, way of dressing, typical cuisine, and so on). A few of these (like Ket, Perrenland, and the Rhennee) are obvious from the published material, others I made up myself (so they're more tenuous and strained and not necessarily compatible with the "canon" of the published products and novels).
This is intended as a very rough and superficial approximation, so it's important not to overdo it and take the approximation too literally - everything is, of course, filtered through a layer of fiction and fantasy. I'm not saying these Greyhawk locales are or should be exactly like earth locales (that undermines the point of creating a fantasy world in the first place), just that I've found this to a convenient short-hand improvisational aid, a way to quickly add some flavor and detail.
Bakluni areas: Persia (further differentiation likely at some future point)
Wolf & Tiger Nomads: Central Asia
Iuz & Horned Society: Russia
Geoff & Sterich: Scotland
Sea Princes: Barbary Coast
Ulek States & Celene: Fairyland from British folklore
Central Flanaess (Verbobonc/Kron Hills/Gnarley Forest/Greyhawk/Cairn Hills/Nyr Dyv/Upper Wild Coast): American midwest
Lower Wild Coast & Pomarj: Pirates of the Caribbean
Bandit Kingdoms: American Old West (Shield Lands = Texas)
Rovers of the Barons: Sioux Nation
Duchy of Tenh: Iroquois Confederacy
Theocracy of the Pale: Puritan New England
County & Duchy of Urnst: Belgium & Netherlands
Nyrond: France (Almor = Avignon)
Barbarian States: Scandinavia (further differentiation (say Frost Barbarians = Sweden, Snow Barbarians = Denmark, etc.) left as an exercise for the reader)
North Province & Bone March: Balkans/Romania
Great Kingdom: Italy (Medegia = Papal States, Rel Astra = Venice)
South Province: Spain (Onnwal = Galicia, Irongate City = Barcelona, Idee = Portugal, Sunndi = Basque country, Lordship of the Isles = Balearics)
Spindrift Isles: Greece
Scarlet Brotherhood: medieval Syria ruled by a secret cabal of Nazis