Donald Duck is well known for his nearly unintelligible speech. Out of universe, this voice was the invention of actor Clarence Nash, and Walt Disney Studios selected it for Donald because the casting director (or in some versions of the story, Walt Disney himself) thought it sounded like a duck.
What's confusing is that the other ducks in the Disney core universe don't currently talk anything like Donald. Donald's girlfriend Daisy, and his cousins Huey, Dewey, and Louie, used to have the same ducky voice, but this was dropped after a few years and the characters now have fairly normal voices. Many other Disney ducks, such as Scrooge McDuck and Ludwig Von Drake, have always had perfectly normal voices.
Has there been any in-universe explanation, according to whatever passes as Disney canon, as to why Donald is the only duck who speaks so unintelligibly? For example, apart from Daffy's throwaway line in Who Framed Roger Rabbit, has any other duck referred to Donald as having a speech impediment?