How many of the places in the games are real places? I know that Jackson Square and Lake Ponchartrain are real (though sadly, you will not be able to hear that beautiful music when you visit the latter). Is St. Louis Cathedral real? St. Louis Cemetary #1? Napoleon House? Dixieland Drug Store? The Voodoo Museum? For the record, is St. George's Book Shop a real place?
Also do people in Lousiana speak like the narrator? Or do they speak like Gabriel? Or do they speak like Mosely? Or do they speak like Cazanoux?
Is the great snake mound in Benin a real place?
Is Rittersburg a real city? Are the background in GK2 real photographs of real places?
Is Rennes-les-Chateau and its surrounding locations real?
For the record, are there really more English speakers in France than in Germany? Every German I've ever met speaks at least 3 languages fluently.
I can only say about the first game. As a rule of thumb, any New Orleans location that displays some year-specific historic information exists in reality. Those include the lake, the cathedral, the square, the cemetery and Napoleon House—all of them are adapted to fit the requirements of a video game locale, as in they're compressed and their tourist presence is significantly downplayed. The museum, Gabriel's bookshop, the drug store, not to mention other pivate estates (that of Geddy's, Magencia's, Grandma Knight's and so on) are entirely fictious.
It's difficult for me to talk about the accent accuracy—very few of the voice cast actually hail from their in-game characters' homelands: Mark Hamill's Californian and Tim Curry isn't even American, for instance. The narrator's probably fairly accurate as Viirginia Capers is from South Carolina. Susan Silo (Cazanoux) plays the part of a Creole fairly convincingly even though she's from NYC and she's actually a fairly prominent voice actress, active till this day.
Just from my limited knowledge on Louisiana in general and New Orleans in particular, it seems that the game highly overestimates the level to which French culture and even the language penetrate daily life there. But someone from the area would have to verify that.
No place like Rittersburg (named after the Ritter family that's of course a game invention in their entirety even though there are Germans named Ritter) exists in real life Bavaria. The game background of Benin is a mixed bag. First off, while the republic stopped being "people's" in 1990 so three years before the game takes place, everybody still refers to it as such and even though that would be understandable in the case of common folk (as in, there are still people who dream of traveling to "Czechoslovakia"), it's a little bit weird a man like Hartridge is still not up to date on political developments in West Africa, seeing as the region is his field.
As for the lore part, the Fula tribe that's mentioned in the game is indeed real whereas the Agris, understandably, are not. History knows of no "Red Basin" there that's particularly nasty: all of tribal Sub-Saharan Africa was as far removed from pacifism as imaginable. West Africa in particular is the spot in which the most advanced African civilizations arose but that didn't really happen until the Middle Ages, parallell to the ascent of Islam to the North. Before that, this part of Africa knew how to make iron but that was about it. There is no snake mould in Benin or anywhere else in the region and we have absolutely no reason to believe there was some sort of great civilization there that became lost to history: African archeology produces an unbroken cycle of events that we can somewhat coherently put together, even if we have no written records from before the Arab period (and only in the Northernmost part of Sub-Saharia) and extremely few of them until the European colonization. There most certainly have been some sun worshipping cults there from the earliest days but that's hardly unique: every single religion on Earth started from a cult of the Sun (I have a theory that's where the custom of avoiding eyesight while addressing an authority originates from: shielding your eyes from the sunlight). So for real life historiography to identify just one of them as "THE sun worshippers" would be a little bit silly.