I think in English this sentence would only be used in speaking about someone's occupation or the application of a skill, so that 'art' is acting as a synonym for 'art-form' or 'knack'. Is this the sense if this sentence in Swedish? Or is this a grammatically correct way to point to a concrete object and describe it as a piece of art?
Perhaps it's regional. "It's an art" is a phrase I use quite often.
"How does the gardener get such straight lines in the lawn?" "I really don't know. It's an art!"
It doesn't mean art in the sense of painting or sculpture, it means something involving a skill or a knack. In the example above, the gardener has mastered the art of lawn-mowing. It's perfectly grammatical to say "It is a skill", and "It is an art" is equally valid.
There's an old anecdote (more legend than fact, unfortunately) that when King Charles II first saw the new St Paul's Cathedral in London, he described it as "artificial, pompous and awful" (or, as a modern English speaker would say, a work of art, stately and awe-inspiring). http://quoteinvestigator.com/2012/10/31/st-pauls-cathedral/
I agree the art of lawnmowing is a very different case. Yes you can use the words - the skill of the artist is an art. (Its clumsy but illustrates the point )
But here we are referring to a particular skill ( an art ) . The more significant use of the word art. To mean that general category of human endeavour that we call art cannot be sensibly used with a indefinite article. You can't look at the Mona Lisa and call it an art.
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More like "it's an art form". But the expression does exist. For instance, BBC (https://www.bbc.com/news/av/entertainment-arts-28895872), The Guardian (https://www.theguardian.com/books/2003/mar/15/julianbarnes.houseandgarden), the NY Times (https://www.nytimes.com/2015/02/08/books/review/is-book-reviewing-a-public-service-or-an-art.html).