"Nobody wants to know where you have bought your sweater."
Translation:Ingen vill veta var du har köpt din tröja.
To express a thought like this in English, I would tend to use the simple past rather than the present perfect: "Nobody wants to know where you bought your sweater".
Buying just one sweater one time is a single event, and the simple past does a better job of conveying that than does the present perfect.
On the other hand, if it were "sweaters" in plural, then there would be a choice of sentences with two different meanings:
1. I know where you bought your sweaters.
2. I know where you have bought your sweaters.
1 refers to a single event at which multiple sweaters were bought. In contrast, 2 is about the purchase of multiple sweaters at different times.
Does Swedish see this differently? Wouldn't the Swedish be more elegant if it used the simple past here for one sweater, and reserved the perfect for multiple sweaters purchased over a period of time?
Either is absolutely fine in Swedish, though I'd tend towards the simple past personally myself. The specific phrase is a reference to a song, though - the existence of which clearly shows that other natives think differently. Or maybe the just wanted something to fit the rhyming scheme - who knows? :)