One reason that some authors say not to write in dialect at all is that syntax and word choice can be used to suggest a dialect, region, or first language without modifying the words whatsoever. For example, in the U.S., specific kinds of pronoun errors are common to more rural areas (also often poorer areas with worse education though not always).
“I got them books,” “They gave it to her and I,” “Clay and me bought two,” etc.
Alternately, putting words in the wrong order is a common way to suggest that English isn’t a character’s first language. Sentence structure can vary greatly from language to language, and small order errors can be telling. With ESL, it’s very common to drop articles and prepositions or to put adjectives in the wrong place (See “The 10 Most Common ESL Mistakes” by Scribendi). Another useful mistake is using the wrong synonym for a word with multiple meanings (“I tried to novel a room at the hotel”) or for more extreme examples, completely reordering the sentence: an example of this Yoda’s speech patterns are.
Actually, Yoda’s speech patterns are more similar to the syntax of languages like Japanese where the verb usually comes last (after the subject and object, which can vary in order). For Russian speakers, on the other hand, it is more common for books and movies to change the syntax to verb-object-subject-verb (“Knows this, everyone does.”). I don’t know much about Russian, so I can’t say how close that is to the Russian language – but it has become the usual dialect tactic.
Of course, if you know a language well, you can better adjust the errors that a character raised on that language might make when speaking English (by following the rules of the first language instead). Otherwise, researching the linguistics might be slower and more painful than paying attention to what kind of syntax and word choice other authors and screenwriters use to suggest that accent or origin.
But wait – there’s more.
Besides errors, you can also use slang, jargon, idioms, and colloquialisms to give an impression of dialect as well as general characterization. The use of “lass” implies Scotland while “cher” is more New Orleans, “sugar” is general South, and so on. Whether a character calls part of the car a “fender” or a “bumper” tells more than we realize. While it’s easy to think of that as more characterization, that type of characterization strongly influences how we read dialogue.
Try imagining “as useful as a milk pail under a bull” in an upperclass British dialect. Now, try to hear “Bob’s your uncle” in a strong Appalachian accent. It’s kind of hard to do, isn’t it? (And yet hilarious).
Since each phrase is strongly associated with a specific accent (because the phrase itself is more likely to be heard in a specific region), you don’t need to modify the words at all to bring that accent to mind. The reader automatically assumes those words are read in that accent.
That doesn’t mean you can’t use the other method. In fact, errors and word choice can work very well with word modification to communicate a dialect without becoming overly complicated or confusing. And with the added characterization, it’s a win-win situation. If you want to show, not tell when it comes to character background and dialect, a character’s word choice is one of the best tools you have.