One of the best examples I can think of where an author uses different dialects in the same story is C.J. Cherry's novel Angel With the Sword (the first story of the shared-world "Merovingen Nights" series). When she writes dialogue for some of the "canal-rat" characters, such as Altair Jones, the apostrophes really get a workout!
When I finish unpacking my books, I'll post a bit of Cherryh's dialogue to show you what I mean. Altair Jones' dialogue is full of apostrophes, but that's to show that she is young (16 years old) and uneducated. She does have basic literacy skills (as in being able to read and write her name), but never went to school, and doesn't possess either books or writing implements. She's too busy running a skip (small freight-hauling boat) on the canals of Merovingen, trying to earn enough money to stay alive, to worry about proper speech. Later in the series, though, another character tries to teach her. He's nice about it, letting her know that she couldn't be expected to know the rules of grammar if nobody had ever taken the trouble to tell her what they are. Most of the other canalers are in the same situation.
Okay, I've found my copy of
Angel With the Sword (it's the one C.J. Cherryh autographed!

). Note that Merovingen is a city of canals much like Renaissance Venice, but the series takes place in the 33rd century. The people of Merovin have deliberately backslid on technology due to fear of detection by aliens who don't want humans on that world.
Altair Jones, a 16-year-old canaler girl, has just fished a strange man out of the canal after he was whacked on the head and thrown in to drown. For some reason she's decided to save his life (unusual, since life is cheap in the city of Merovingen, and Altair is part of the seamy underside of the city, being engaged in the smuggling trade). She has no idea who the man is, just that he's older than she is and "pretty".
After he wakes up, she's trying to find out what his name is and why the others tried to kill him. He's slow to respond, so she asks:
C.J. Cherryh said:
"You got a voice?"
"What am I doing here?"
Not what'm'i'doin' ere. Clear and pure as a voice could speak it, a quiet, immaculate voice that brought her and her outheld hand to a frozen stop.
She heard that kind of accent at distance, at the distance of lordly voices drifting from the heights of bridges and the insides of buildings and the other side of grilled doorways.
This is a small example of the distinctiveness between the upper-class speech and the canalers. There is a middle class in this city as well, and they speak much like any middle class person would - mostly educated, with occasional slang, but not the patois and cant of the underside.
I may or may not know what you're talkin' 'bout. <----If you mean apostrophes for a purpose (here, to connote uneducated speech) then I have no issues with it.
That's exactly what I mean.
And for the writers who
do make up an entire languages, well they had a choice to add in all those apostrophes. They choose poorly. Everything but actual Klingon needs to go.
I used to play a lot of Scrabble with a friend. We took a close look at the rules and realized that they don't forbid non-English words. So we decided any language was fair game as long as we could prove the word really existed, and it didn't violate the "no capitalization" and "no punctuation" rules.
Our games ended up a mixture of English, French, German, Swedish, and Latin (since those are the real-world languages we're most familiar with)... plus made-up languages. I have a very basic Vulcan dictionary and some word lists of Klingon and Romulan words. The problem with the two latter languages is that nearly every. damn. word. has a capital letter thrown in for no reason I've ever been able to fathom.
I also think that many of the authors who use this don't even intend for the apostrophes to mean 'glottal stop', they just think it looks cool/alien/exotic/whatever.
Marion Zimmer Bradley did this is the early editions of her Darkover novels (from the early '60s). She wrote "Comyn" as "Com'yn". There was never any good reason for it, so she later dropped the apostrophe.