There's certainly a "problem" with the first lines even to modern-day Scottish people, but then, language does change and evolve over time. "Is there", without anything more, was quite acceptable usage in Burns's day for "Is there anyone who". I'll offer first an expanded version of what I'm pretty sure RB was getting at in these lines, prefacing it by stating that I recall reading (somewhere) that an earlier draft of this reads, "Where's he [that] for honest poverty &c".

"Is there [anyone who], because of his being poor, hangs his head? By this, I don't mean those who are literally slaves - that's another thing entirely, so we "pass by" this question.* We dare, we are prepared, we are not ashamed, and are even proud to be poor."

Two other things which may be of interest; it's been several times pointed out that some of the thoughts in this song (about "ribband, star, and a' that") correspond closely to ideas in Thomas Paine's "Rights of Man", and indeed the lines about "A prince can mak a belted knight...&c" evidence Burns's familiarity with the works of many English poets - something about "earls and knights are but the breath of kings/An honest man's the noblest work of God", which I think is Pope.   Secondly, the expression "an' aw that" (anglice, "and all that") is in common usage in contemporary Glasgow, at least. There are, by the way, other examples of verses set to this air, sometimes by Burns:

"I am a Bard of no regard wi' gentle fowk, an aa that,
But, Homer-like, the glowran byke frae toun tae toun, I draw that"

(A character in "Love and Liberty"/"The Jolly Beggars", as it has been retitled) sings that he, like Homer, draws the ordinary people to listen and to gaze upon him as he wanders from town to town). Hope this has been of some use, as well as some diversion.


*[as well, I suppose, as the more obvious image of "passing by" a slave, or perhaps a mendicant)