Which brings up a point- in the comics, Donald appears to speak without any sort of impediment, and is also quite eloquent. In the cartoons... not so much.
It's probably to do with it being easier to add in speech than in writing. But I'm sure someone on YouTube can make a 10 hour documentary about the deep lore behind it!
For anybody unfamiliar with Barks and his Duck-legacy, look him up on wikipedia, or better yet: Read his classic Uncle Scrooge stories. Best work he has done!