FIRST, DO NOT CREATE A DIALECT WITH THE SAME KEY AS AN EXISTING BASE ONE, I DO NOT KNOW WHAT WILL HAPPEN, I DO NOT CARE TO KNOW, JUST DON'T DO IT! IF YOU WANT TO MAKE YOUR OWN VARIANT OF THEM, JUST CALL IT, IDK, GILNEANTWO OR SOMETHING! THESE ARE THE BASE ONES:
gilnean, dwarvish, draenic, troll, orcish, noble, poor, pirate, darnassian, thalassian

Create a file named Custom.lua.

Paste this example in:

CustomDialects = {
    Pirate = {
        ["hello"] = "ahoy",
	["steal"] = {
		base = "nick",
		["stealing"] = "nicking",
		["stolen"] = "nicked"
	},
    }
}

Now I'll explain it portion by portion:

CustomDialects = { - opens the code
    Name of the dialect = { - name of the dialect (single word)
        ["single word to translate"] = "new word", - single word translation
	["single word to translate"] = { - works the same as above, except you can add other words inside it, for better organization
		base = "new word",
		["another word 2"] = "new word 2",
		["another word 3"] = "new word 3"
	},
    }
}

You can use only single word translations or just groups of words.
Don't forget to end every entry with a comma.

Example of 2 dialects in the file:
CustomDialects = {
    Pirate = {
        ["hello"] = "ahoy",
    },
    Noble = {
        ["hello"] = "Greetings",
    },
    Commoner = {
        ["hello"] = "hullo",
    },
}
