Really, I do.
Not that I claim to have a great understanding of the japanese language, but I've heard from plenty of japanese-understanding and -speaking people that "Pirateking" is a more correct term.
What I dislike even worse is people who claim them to mean the same thing. While a "Pirateking" sounds like a rank (x is an admiral, y is a clown and z is a pirateking), "King of the Pirates" sounds like it means exactly that the holder of he title is a monarcial emperor over the pirates, just like "King of sweden" is the monarcial emperor of sweden. Swedenking (or Swedeking) would just be a fancy title though, and sounds like something cheesy given to winners of an annual skiing contest or something.
And it's also pointed out to be false in the manga. When fighting Enel, this conversation takes place (possibly a loose translation, but I think the message is quite clear):
Luffy: "I'm the man who's gonna be pirateking!"
Enel: "Pirateking? And what is he the king of?"
Luffy: "He's the king of sea!"
Enel: "Interesting. Let's finish this in the sky."
Luffy explicity states that the pirateking has nothing to do with being an emperor over pirates, but merely being a "king" (I believe master would be more appropriate) of the sea, basically a title to respect, not planted in political power or the need to be obeyed.
And I also dislike the swedish translation of "Smoker" to "Bolmare" (to "bolm" is to puff, like "röken bolmar (ut ur huset)" is "The smoke is puffing (out of the house)". That's a whole other rant though.