Lastly, and the only important thing - re the song. It's a very tough call & nobody yet has solved the problem to my satisfaction. I love & sing a large number of Scottish songs. Unfortunately, where I sing not a single word gets understood by anybody. That includes whether I translate all dialect words to standard English. All conspire - the literary vocabulary (even translated) the meter, the accent, the narative type and even etc. Certainly it is the job of the communicator to do the communicating but in this case I fail. There are many excellent rendered-in-English Scots trad songs (say, Fairport's "Tam Lin" or any Martin Carthy effort) but something is still lost, I believe. In Ballad Scots it's much easier as the vocabulary and context and subtilties (if not the story) have become simplified. It hardly matters & hurts the story not at all whether you sing 'kirk' or 'church.'
But the more dialectical songs - those that play with the language, itself or 'rustic' songs such as most bothy songs - there's a difference. They don't easily translate. The context & humor & puns are just too different. Here the saw that for a translation to work, the translator must be at least as good a writer (or poet) as the original. How else can the context or subtilties come across?
For "Farewell to Sicily" I'd likely tend to agree with your Scottish friend. I think the words in DT are from (or also from) the Chad Mitchell Trio stuff. It's an honest enough attempt at a singable rendering of the song. It's certainly nothing close to literal and I believe it overlooks too much of the meaning.
Hamish isn't a "folk" - source singer - he's a great poet and an amazing savant of Scottish material. The article from Chapbook cited by Gaughan is very worth reading - even if I'd disagree with a few points and some of its polemicism. Hamish tried to combine a feeling of Scots language but he does not pick a particular local & stick to any local word usage - he carefully chooses his words from all over the country. He conveys a subtle love/hate of Sicily - Whaur kind signorinas were cheerie and Nae hame can smoor the wiles o' ye - the landscape is as fine as home and also, BTW, the people are very similar to the Scots in life, poverty, humor & discrimination by a controlling "foreign" (ie, Italy) government. And where also the Gordons experienced months-on death, exhaustion, frustration.
But that language is intended to convey how the Scots felt about it - not how any others would have felt in similar circumstances.
Do I mean that one shouldn't be 'allowed' to sing such a song outside Scotland except in the (generally incomprehensible) original? or that one must first hire the world's greatest poets to render it? (Well, maybe ys - Thompson hired Beethovan & Haydn to render the tunes, after all.) or what do I mean would be the best & most honest way to communicate such foreign-language song to an audience? - translate? half-translate? translate text but not accent? stick to the original (assuming your tongue will allow it)? explain in detail? or even explain line by line?
Hell, I'm not smart enough to answer that one & I've been trying 45 years.