I just remembered an example of what I really hate--although it is a poem/reading instead of a song.

The famous poem by Robert W. Service, "THE CREMATION OF SAM McGEE" begins:

There are strange things done in the midnight sun
By the men who moil for gold;
The Arctic trails have their secret tales
That would make your blood run cold;
The Northern Lights have seen queer sights,
But the queerest they ever did see
Was the night on the marge of Lake Lebarge

Several years ago, I heard this read on the radio by a well-known local story teller. He got to the last line of that introductory verse, and changed 'marge' to some weak, silly, vague word (I can't even remember what he used now) because, I assume, he thought we poor listeners wouldn't know what a 'marge' is!

It makes no difference whether we know what a marge is, THAT can be looked up if it is at all important...the man wrote MARGE!

Similarly, various people try to re-write Hamish Henderson's "The 51st Highland Division's Farewell to Sicily" because it is written in dialect!

"The pipie is dozy the pipie is fey
He wullnae come roon for his vino the day
The sky o'er Messina is unco and grey
And a' the bricht chaulmers are eerie"

fine! It takes a wee bit of effort to get the references, but it 'feels' right in dialect, and changing it does a disservice to the intent. Henderson could no doubt have made it an English song if he had wanted. I feel that mostly, if you can't do the language, do a different song. Sometimes, even in Scots dialect a slight word or two change, if they are really coherent, can be ok, and help the flow....but not when it alters what the song is about.