I will put in the occasional cuss word, but for the most part, I write in worlds and times and societies where such language is either nonexistent or not very classy, which it isn't in any world, if you ask me. Webster went to great lengths to write a dictionary; some of these people should take a look at it once in a while, they might actually be able to get their point across better. Okay so now you know how I feel about cussing, and it has nothing to do with religion; it's just stupid. Rant over. Please accept my apologies.
No, my writing is not all lilly white, but I strive for my words to be as invisible as possible so the story can come through, and whenever your reader has to stop and try to figure out what someone said or what is happening, that effort has failed.
There's another aspect of this. Accent. There's the southern accent, the New York accent, the northern accent, the Texas accent. Those are the ones I can think of in this country, and then there's the foreign accents of people coming to this country from wherever around the world. And of course, if you have an extra-global society, there's that accent too. Spelling these accents can be very distracting. My advice would be to keep it simple. Too many tweaks on the spelling, and once again your words are getting in the way of your story. In reality, most foreigners probably speak better than the average American.
Spelling isn't the only cue to a different language foundation. Sentence structure is a biggie.
“If no mistake have you made, yet losing you are … a different game you should play”
Yoda is probably the most famous of those who twisted sentence structure. If you've ever studied any other language, that is much closer to the norm; ours would seem twisted by comparison.
So, how do you show where your character comes from, or do you bother? For me, most of the time, it's one of those get-to-know bits of information that's introduced somewhere along the line. I seldom use accent spelling, though I might use dropped endings replaced by an apostrophe most of the time. Most of the poor or low class people in my books have little education and so talk like their parents did, and probably for generations before that, but they would never dare to be so rude as to cuss openly shy of what might come out if they smashed a thumb with a hammer. There's always those kinds of incidents to watch out for. I've nothing against a little spice now and then.