There are many reasons why a writer needs a character to speak a certain way or with a certain accent. In fact, the inability to understand a character may be an important part of the story.
I once read a screenplay where the writer had the characters speaking in either a Scottish or hillbilly fashion – honestly, I couldn’t tell – but instead of simply stating it with dialogue, action or in a scene description, he wrote out every word.
Ello, do yous haf the time
Imagine an entire screenplay riddled with language like that from several characters. I had no idea what kind of accent they were using or why they were saying it that way. When writing a spec script, you want it to be easy to understand for the reader. They will never get past the first page if they have to spend five minutes deciphering the dialogue.
Readers literally have dozens of spec scripts to look through every day. Your script has to be interesting and easy to read, because you only have about 10 minutes of their time before they make a decision to move it to the next level or toss it in the trash.
Every word of dialogue should be important to the story. If a word is changed in the dialog, then there should be a reason. For example, a mispronunciation of a word causes an argument. Otherwise, it should look like standard English.
Instead of mucking up your screenplay with accents and local vocabulary, use parenthesis to make your point.
(thick Scottish accent)
Hello, do you have the time
You can also include it in the scene description.
Ext. Grocery Store. Night
Two Scottish cops stand outside the scene of a grocery store robbery. Each smokes a cigarette impatiently waiting for the coroner.
You mention that the police officers are Scottish, so it is implied that they have accents. You can also make it more explicit if needed.
Ext. Grocery Store Night
Two Scottish cops stand outside the scene of a grocery store robbery. Each smokes a cigarette impatiently waiting for the coroner. Carmen has an almost incomprehensible speech impediment.
I cannot stress the importance that this should only be used if it is absolutely necessary to your story. If the character’s accent or impediment doesn’t add to the plot, story or character, then leave it out. Screenplays may only be a hundred or so pages, but they have to be streamlined. There is a lot of information packed into those pages, and none if it can be superfluous.