Having grown up in south Louisiana, I can tell you that Cajun French is to French what the English spoken in the hollers of Kentucky and West Virginia is to the Queen's English.
Within that red triangle is "Cajun country." My deer camp is approximately where the "N" is in Louisiana on that map in West Feliciana parish, about a mile or 2 east of the Mississippi River. The culture there is indistinguishable from redneck Mississippi, just 20 miles north of my camp. But go across the John James Audubon bridge into Point Coupee (pronounced cuh-PEE) parish and you're in an entirely different world. In West Feliciana, surnames like Kinchen, Ard, Harvey, Brannon, and Richardson are common. Just across the river in Point Coupee, you'll find surnames like Landry, Boudreaux, Shexnaydre, Guyran, Arnaud, Simon (it's French, pronounced "see-MON" with the "N" pronounced in the standard French glottal fashion, not with the tip of the tongue in the front of the mouth as would be the English pronunciation).
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Of note, neither Baton Rouge (now the largest city in Louisiana) nor New Orleans are in Cajun country. Also, you'll find a lot of people in that area who have lots of Italian and Spanish ancestry. Many folk consider New Iberia, Louisiana (Iberia/Spain???) more or less the heart of Cajun country.
In the center, just above the tip of the triangle, is Alexandria. As redneck a place as ever you'll find. Just a few miles southeast from there, perhaps 10 miles, is Marksville. Again, it's like going from my place in W Feliciana to Point Coupee - a complete culture change from Alex. to Marksville and Mansura.
Outside the triangle, religiously you'll find Presbyterians, Methodists, Episcopalians, and Baptists and other evangelical churches, with a smattering of Catholic churches. Inside the triangle, you'll find a small statue of Mary in front of many/most of the houses, and pretty much everybody is Catholic.