Some interesting views and food for thought in this sub-thread (which now has nowt much to do with MacColl himself).

If I like a song well enough to want to sing it myself, I will do my best to research its background, and if necessary learn to speak (as well as I can) the appropriate accent and dialect. As I said above, this is no more difficult than learning to speak a foreign language, which many people do and nobody questions it (although I recognise that not everyone finds learning languages easy). I will do this ideally by spending time in the area, but more likely I will talk to people I know who are from that area, listen to speech recordings or recordings of singers from the area, etc., and above all practice speaking/singing to get as near to the 'correct' pronunciation as possible. In other words, I work at getting it right, as much as I can.

If I then sing the song and don't get it 100% right, and someone from the area in question gets offended, I would regard that as their problem not mine. I've found that most people appreciate the effort I have made to 'speak their language', just as I find when travelling abroad that saying things like "hello", "please" and "thank you" in the local language (even when you don't know many other words in it) goes a long way towards getting along with the locals.

As for dialect words, I always explain them in the introduction (unless of course I'm in the area where the dialect is spoken), as well as other things like place/street/people names that might have a bearing on understanding the song.

I don't think that not actually coming from the area should be any barrier to singing a song from that area, providing you make the effort to get it right. As I said, most people appreciate this and if you get the odd person who somehow feels insulted and thinks I'm taking the mick out of their accent, then that's their problem. Again you can probably pre-empt this by saying something in the introduction to the effect that "I don't come from the area as you can probably tell, but I'll do my best... blah blah blah... as I don't think it would sound right any other way".

Yes, there is a case for "translation" but what you end up with is not the same song at all. And as others have said, if you do that then you might be accused of arrogance - you probably can't please everybody, but since when was that a reason not to do something?