I'm with John on this - either find an English version of the ballad (far less likely to have dialect words) or rewrite some of the more archaic Scots e.g. 'Ye'll get twa sichts o your leman' = You'll get two sights of your sweetheart/true love.

I work with a group of people who want to sing ballads, but have exactly the same problems as you have -- our bottom line is to understand the story first of all, find a set of words that you like, find a tune that seems to be a good match for the emotion of the story..... and then comes the biggest job which is to tweak the words and/or the tune so that the text sits tidily on the melody and sounds like the way you would speak it, with emphases in the proper places.

Hope that doesn't sound too daunting, because it's very rewarding when it works -- but it's a very different approach from classical music, where the composer has determined pace and dynamics in advance and has matched melody notes to text syllables in a way that can't be altered.

Good luck with the project.