I agree the business of deciding whether to try to keep the original form or sing a translation of a song is tricky. I don't have any hard and fast rules myself, but I do feel that the original usually has cadences and other poetic structures that are very difficult or impossible to maintain in a translation. As a result I would normally try to keep close the original if I can. The problem is not just Scots songs of course. I have always preferred "Adeste Fidelis" to "O come all ye faithful" - the translations of the second verse (God of God/Light of Light/Begotten not created) is almost impossible to do well in English (drag-it-out, drag-it-out, cram-as-many-syllables-in-as-we-possibly-can is the usual form!). The same applies to old English and middle English to modern.

But I agree there does come a point at which the singer and audience have lost so much understanding of the original that your only choice is between using a translation and dropping the song altogether.