Not all bad guys use bad grammar. Some are very suave and sophisticated. Similarly, not all persons doing low-paid jobs use bad grammar either.
Every person from Scotland does not use “wee” or “drap” in every second sentence, nor does every Australian call you “mate”.
Typecasting happens because there are similarities inside groups. An Australian is far more likely to call you “mate” than a British person is, but both of them are more likely to use your given name.
Please don’t typecast your characters. If that strong young man on the road crew drops his mallet on his toe he probably will swear. But then, so would a doctor or lawyer. And all of them are equally likely to say invite (verb) instead of invitation (noun) because they’re probably all on Facebook. But there is no excuse for bad grammar in narrative. Nor is there need for endless “local color” in dialogue. The lady in the Scottish hotel may offer your hero “A wee drap” of something alcoholic to drink. But leave it there. Don’t overload your story with colloquialisms or poor grammar.
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