I don't think in this case it was ths Scottishness of the language that was perplexing some readers of the poem/song. it was just that first line, which is no less perplexing in English.

To get back to the language issue: I can't see, Joe F, that "let who will be clever" is a similar ellipsis. It's quite normal for "will" and "want" to be used elliptically, even in modern English, e.g. "I will if you will" or "You can try it if you want". It's not normal at all to say "Is there who understands Scottish?" or "Is there who agrees with me?" I do understand it now it's been explained, but I'm not in the least surprised that the question was asked in the first place.

Marje