My D&D campaigns and the real world cultures that exist within them are perhaps an unconscious homage to D&D's “The Known World” campaign setting that was introduced in the Expert set I got pretty much as soon as it came out. The Known World (later AD&D-ized as Mystara) was the first world for D&D I ever saw and even as a kid I could tell that Ostland, Vestland and the Soderfjord Jarldoms were Norse lands, that Ylaruam was Arab and Ethengar was Mongol. Sure the Five Shires were a Tolkien rip-off and the other racial homelands were just kind of tacked into place. I also didn't get the other historical human cultures at the time, somehow even missing the fact that Thyatis was just Rome reskinned for this world.
I am not really sure how it happened that my Kingdom of Garnia campaign world turned out so much like the Known World. When I started writing stuff up for it I was consciously aping the World of Greyhawk in format, because it was AD&D and I had moved on from B/X. I guess it started when I gave my world an origin mythology. I wanted to have a world where the people made sense, logic and reason were constantly being thrown at me from all the campaign design articles I was absorbing from the Dragon. So I got a real world culture, the Gaels who are near and dear to me only because my family is of Scottish Highlander origin, and I made them the centerpiece culture for the setting. The were the “People of Ailill”, the semi-legendary leader that brought them to this world to escape the destruction of their culture on ours. Their Druids could open extra-dimensional portals through the use of planar gates constructed in ages past by the Elves. These portals took numerous forms, but were mostly rings of standing stones. Like Stonehenge.
As I learned more about history, particularly the history of the Gaels, I retconned the setting to make it have more and earlier Celts. Celts fleeing Roman aggression, Celts fleeing encroaching Christianization, Celts fleeing inter-tribal warfare, Celts escaping from German aggression, Celts escaping from cultural assimilation from Asia Minor to the British Isles. This may have been partially resulting from a piece of D&D campaign advice I read, probably in the Dragon, where the author said to use language to give your world verisimilitude; something like “if you have a string of border forts all named Dun Something, your players will eventually figure out that Dun means Fort”. That struck a chord with me, because I knew that Dun was Gaelic for fort, and I really was into the whole Scottish Highlander thing back then. It was maybe the better part of a decade after “Roots” had aired, but Americans were still pretty interested in learning theirs and my Grandfather had taken a real interest in making us aware of our Highland heritage. We were going to the Scottish games every summer, listening to bagpipe music and Scottish folk songs. I even got a kilt, which I wore to school on numerous occasions; for the record chicks dig guys in kilts, guys call it a skirt and try to mock you. I liked the attention from the ladies and the challenge from the guys, fornication and brawling are both lifelong hobbies of mine!
Anyway, my campaigns have included both kilt wearing claymore wielders and plaid-trousered head-hunters; immigrants to Garnia from over a millenia of Celtic movements. Officially, by the way, Garnia is Gwarynica Riga in their language. The dominant language of the campaign area is a hodge-podge of Gaulish, Welsh, Gaelic (Irish, Manx and Scottish) and Breton, roughly in that order. I discovered Katherine Kerr's Deverry novels in the mid-1990's, introduced by my then girlfriend- now wife Mona, and I have used some of her Deverrian language to fill in the blanks in a few spots since her and I were working from the same starting point. I have worked hard to not cross pollinate my world with hers, but some was probably inevitable after I discovered her writing.
The earliest non-Celts to make the journey to Garnia were the heathen Anglo-Saxons. They wound up with their own kingdom at the edge of the mapped world almost by mistake. When I started adding realism to my world in the form of real human cultures I started by making them Germanic. I Germanized the entire world, but I didn't like the result. So I de-Germanized the world and Gaelicized it instead. This happened at a key time. I suck at making maps, and I don't particularly like it. I was in 7th grade and my BFF Darryl wanted to help with the project, since it was pretty much all we talked about at lunch. I tend to get a little single-minded when I am working on something and my enthusiasm for a project can be infectious. I gave Darryl some of my material so he could make some maps for me. The bulk of the map I gave him had been altered, but the Wotanic Knights (think heathen Teutonic Knights) were still there at the bottom of the map. I liked the results of his mapping, so they got to stay. I eventually gave them their own origin even.
The Basques made it into the world next. I liked their shadowy pre-indoeuropean origin, so I had the Elves rescue a bunch from Celtic encroachment and give them a place to stay while they looked for a new world for them. The same treatment they gave the Neanderthals in the canon history of the campaign.
Arabs and Turks made it in later, in the late 80's or early 90's for the sole reason that we had some Arab and Turk looking minis. Previous to that I had exactly one Arabic sounding character in the world and he was a powerful mage (necromancer actually), so he could have gotten there on his own. Al-Khalid was just a name I picked out of a hat, basically. But in my long running Dempster: Phase 2 D&D campaign, Arabs and Turks started making regular appearances because of those minis. Hakim the thief was a party henchman and became a huge catalyst for adventure in his own right when he betrayed the party and made off with a flawless fist-sized magic diamond. Ivar , the party's PC thief and Hakim's former boss, tracked him back to his own homeland and attempted to get his revenge. That was easier said than done and gave us an entire campaign arc set in my pseudo-Arabia and eventually led to Ivar taking on Abdul the fighter as a regular Henchman. Turkish mercenaries were a small but significant part of my last AD&D2 campaign in Garnia circa 1999-2001. That campaign had a lot of stops and starts and player changes and ran concurrently with my Mighty Celts campaign that ended with the first colonization by Celts into Garnia.
In the mid-1990's I ran an AD&D2 campaign set in a fantasy Roman empire, so that eventually became a part of the world too. I placed it on the west end of the large continental mass that included Garnia, Frodia and the rest, far from the old campaign areas in what is an approximation of the Mediterranean world. That area also included an old fallen Egyptian empire, where the ancient Egyptians had been completely overrun by Goblins over the course of centuries. The Goblins there had almost completely adopted Egyptian culture and religion and language, giving us a super-civilized Goblin empire, complete with a Goblin Pharoah. Of course the Romans conquered the Goblin Egypt, and non-human slaves were pretty common in that setting. That game didn't run long, but it was interesting.
It was in this era that I started making my own maps. I used the map-maker for Civilization 2 and added the new stuff right on to the western edge of the old maps. I placed the Civilizations on the map and watched how these areas actually interacted with each other in game. It was an experiment that I found quite helpful for rewriting great chunks of the canon timeline. It also influenced me to add a few more cultures into the mix.
The early part of the 2000's, right after the release of 3e was an interesting time for my campaign world. I ran an entirely new viking setting when we started playing 3rd edition, that setting soon became part of Garnia too, off in the southern sea. Now the Vikings were here too. Around 2005 I decided that it would be cool to add in some of the civilizations from my Civilization 2 map, this coincided with me finding out that one of my country's names translated into Chinese was Ming Liang, right where I had placed China on my Civilization 2 world map. Obviously that was a sign. So I added the Zulus too, after realizing that I didn't actually have any black people in my world. The Chinese made me think of an old Oriental Adventures campaign I'd run in 1996-97, so my Japan cognate Tenchuko got plunked down off the main map, but near enough to Ming Liang to make all the stuff that happened there canon for my world too.
After that I got it into my head that, since it was fun to see the cultural interactions of wildly different cultures from across history play out in my campaign world (Civilization series fan here, Hello), I'd add in a few other cultures that have maybe died out or failed to thrive in our world that I would like to see there in Garnia. I sprinkled in the Nubian kingdom, the Mongols (I couldn't leave the inspiration for my beloved Steppe Warriors gaming guild out), the Kung San, Vietnamese Montagnards, the Aztecs and a few others; then I placed them near groups that they were culturally dissimilar to and mixed things up.
The only downside of this cultural/historical experiment is that I then don't get to use these cultures as touchstones for my demihuman and humanoid cultures like so many others can and do. I have worked hard at making my Elves have a unique and somewhat alien culture, but I still need to de-nordic my Dwarves. That would probably be more of a priority if anyone wanted to play a Dwarf. Gnomes fall into the category of their cousins the Dwarves, but less so. Halflings I have a really hard time not seeing as Hobbits. I work at it by looking at the cool illustrations of Halflings from early D&D stuff like module A1 or the old illustration from the Expert Book, tiny bad-asses rather than Tolkien-esque Hobbits.
The Orcs get a slightly different treatment from me. I always picture them as somewhat like Next Generation style Klingons. The Orcs even got their own Garnia adjacent kingom. Hobgoblins get to be somewhat Asian inspired, owing to the illustration in the Monster Manual. Goblins and Bugbears are something like less evolved or devolved Hobgoblins, like the forces of evil only got the mix right with the Hobgoblins. The Bugbears are their less evolved brutish cousins and the Goblins are their degenerate spawn. Trolls are just downright alien, they might as well be the aliens from the Aliens movies for all that we can fathom their culture or their motives. Giants are a tough one for me because I always want to make them like the Fomor and instead wind up with the Jotnar, score one for our Germanic Anglo-Saxon culture over my desire to make a more Celtic world, eh? Dragons are usually pretty alien too, reptilian but owing a lot to Beowulf or Fafnir, I know Germanizing again.
Kobolds get to be little dog-men just like it said in B2, Gnolls are hyena-men; no need to add a lot of culture there. Lizardmen are tribal reptiles, I always just play them as generic primitive tribesmen and have never had a problem. Pretty much every other monster is either rare enough that building them a culture would be a complete waste of my time or hasn't appeared in my campaign (yet). I like the idea of rare or unique monsters, probably as a result of my years of Conan comics, novels and short stories. REH and Marvel comics probably had more to do with how I made my world than Tolkien ever did, despite the Tolkienisms of pretty much every D&D world ever made. Like the great EGG, I have vastly preferred a humanocentric world, which is why my map looks more like Hyboria than Middle-Earth.