April Fools directly translated to Swedish is April dårar, yes, but it's not the term used when you prank someone on April first.
The short way of telling someone that it's just a prank is screaming "April, April!" in their face. That's the super short version of this sort of verse or nursery rhyme or whatever which has a long and a short version.
"April, April jag kan lura dig vart jag vill" (translated: "April, April, I can fool you in any way I want", though the direct translation is I can fool you everywhere I want or to whatever place I want)
"April, April, jag kan lura dig vart jag vill, din dumma sill" (translated: "April, April, I can fool you in any way I want, you stupid herring", though it rhymes in Swedish)
Some people use the saying "Maj, Maj Måne, jag kan lura dig till Skåne" which translates to "May, May Moon, I can fool you all the way to Skåne". That saying doesn't make much sense about April fools though. Skåne is the part of Sweden furthest down south, which used to belong to Denmark and my personal opinion about Skåne is that the people there are half danish. They all talk like they've got porridge in their mouth.