Futility Closet does a good job of explaining what the poem means and why it’s supposedly untranslatable. The explanation isn’t long, and I’d rather send you there than repeat it:
As it says, would be hard in Spanish, which is similar to Portuguese, to create a close, literal translation that preserves the tight rhyming scheme, and impossible in English. But translations don’t have to be close and literal. They can be idiomizing and even free. For a difficult translation, rather than word for word, a good technique can be to aim for equivalence, which means the translation would have the same effect on the reader as the original, although it may vary quite a bit from the original.
For example, fairy stories in Spanish end “y fueron felices y comieron perdices,” which literally means “and they were happy and ate partridges.” The equivalent in English is “and they lived happily every after.” The literal translation makes no sense, and the equivalent translation would be the correct translation.
Here’s my equivalent translation of “Succinct Serenade”:
— Sue Burke