The answer is it that takes hardly any effort for a Swede to learn Danish and how to speak it - because Swedish and Danish are about as closely related as the deepest Glaswegian and the oity-toitiest Surrey-speak - with a few hundred idioms and special words thrown in to keep things more interesting.
A knowledgeable Swede once told me that the Danes swallow half their words and mangle the rest!
And I agree! Although I can read Danish with ease, I long since gave up trying to get my poor old epiglottis round Danish pronunciation.
But so close in fact are the three Scandinavian languages (Norwegian, Swedish and Danish) that the Scandinavian airline SAS uses Norwegian as their official language and that causes absolutely no problem for any staff be they Swedes or Danes.
How difficult it would be for an individual Swede to learn Danish depends entirely on how much of a 'musical' ear that individual Swede has - and also on how daft that Swede is in wanting to do something so barmy as wanting to learn such an unmusical language as Danish.