Why do Chicagoans, or everyone for that matter, pronounce Des Plaines, “Dez Plainez” - not “De Plain” (like “De Moin” for Des Moines)?
I tried searching through the message boards but my browser is screwy…
People in St. Louis aren’t much better when it comes to pronouncing French names either.
And it isn’t just French. Ask someone in central Illinois to pronounce New Berlin, located about 20 or 30 miles west of Springfield. It isn’t pronounced like that city in Germany. Nope, the accent is on the first syllable and the second syllable is pronounced kind of like the vowel sound in “duh.” And this is in an area that has a large population of German-Americans. I’m guessing the pronunciation change happened right around one of the world wars.
Here in Missouri we don’t pronounce Versailles, MO right either. People say Ver-sales.
But the small town of Aux Vasse, MO seems to be pronounced pretty close to correctly.
How is that pronounced? Ah/Oh Vahs (sharp “s” at the end)
Aux Vasse should be pronounced (in French) Oh v-ass, with a flattish “a” and a soft (unvocalised) “s”.
I wish everybody knew the IPA. It makes life so much easier.
This is endemic in the Midwest. Here in Michigan we have Milan (pronounced MY-luhn, rather than the correct Italian mih-LAHN), and in Ohio there’s a New Paris pronounced like the New Berlin above (with the accent on the New, and pronounced like one word, “NEW-perr-iss”.) We Midwesterners seem to enjoy bastardizing foreign names, for some reason.
Personally, I pronounce them correctly, mainly to annoy the locals. 
LL
It would also be easier if all of the IPA characters were readily typable.
(And /s/ isn’t vocalic, /z/ is. What do you mean by a non-vocalic “s”?)
LL
Well the locals don’t pronounce Aux Vasse perfectly correctly, but they don’t pronounce the x or the e, which is pretty good compared to Des Plaines and Versailles. For this I am willing to grant some credit.
On a personal note, I grew up in Springfield Il and went to college in Columbia MO. One route between the two used to take you through the tiny town of Aux Vasse. I was curious to know how the locals pronounced it. One time my mother and grandmother picked me up from school to take me home for winter break. I casually mentioned my curiosity to which my grandmother pronounced it and told me she was born there. Up to that point no one had even bothered to tell me that she was originally from Missouri. I have lived here a total of 11 years although not all at once, and have since verified her pronunciation.
Once in chat, I told someone that I lived in Des Plaines. He said, “No, you mean Des Moines.” What a loser.
Well, I DO live in Des Moines. We may pronounce that name correctly, but not others. Around here, we have a town named Nevada, pronounced ne-VAY-duh. Also closeby is a town called Madrid, pronounced MAD-rid. It makes it easy to spot folks that aren’t from around here.
Yeah we have a Nevada, MO pronounced the same way as Nevada, IA. Maybe those people out west pronounce it wrong. I remember people I worked with who were in Ames telling me they lived in Nevada and being surprised at the common (mis)pronunciation.
You might also “Pierre” into the reasons behind South Dakotan pronunciation.
I honestly don’t know if they are more or less “correct” than the expected pronunciation. St. Pierre, as in the island, seems to have three accepted variants.
There’s also My-am-uh, Oklahoma, spelled Miami.
And another example (if you’re not sick of them already): The University of Idaho is located in Moscow, Idaho, but it’s pronounced MOSS-koh, not Moss-KOW.
Displaced Chicagoan in Omaha, NE now. I miss it. 
Anyway, town of Norfolk NW of Omaha, is pronounced here as NOR-FORK, seemingly accenting both syllables. Must be the bio-engineered corn they put in the chips for the Huskers’ games. Man, I can’t stand them. But that’s another topic…
Vis
I think you’ve hit it on the head. I can’t swear to the truth of this, but I was told by someone who studied the history of the area that the pronunciations (at least some of them, like Milan and Berlin) were changed during WWII. The natives didn’t want to be associated with those places in Europe.
From what I recall, Cairo, Illinois was founded during the Victorian boom in Egyptian archeology. However, they decided to call it KAY-ro (instead of KI-ro) to remain unique. Go figure.
Another oddball is the Arkansas River which runs from Colorado thru Kansas, Oklahoma and Arkansas before joining the Mississippi. In Colorado, Oklahoma and Arkansas its pronounced like the state name, Arkansas, but in Kansas, its pronounced ‘ar-Kansas’.
The French bakery/restaurant Au Bon Pain. It’s pronounced “oh bon pon”. But just to bug my wife I pronounce it (with a nerdy Jerry Lewis voice), “Ow! Bon Pain!”
I spent 14 years in New Orleans. The pronounciation of that NAME has really been bastardized. Most folks go with “Norlins” or “Nahlins.” You try saying “New Orleans” and they’ll look at you funny.