I can't read japanese so the way I see it is this:
1- If WSS is a true translation then Mihawk is the strongest swordsman regardless of your skill lvl (Kengo, Dai Kengo, etc...). Meaning he's stronger than Shanks and all other swordsmen like Vista, Shiryu, etc...
2- If WGS is a true translation then Mihawk wields the best swordsmanship in the world. It's not a title related to strength. He could be the weakest swordsman ever as far as we know but still has the best swordsmanship in the world.
3- If WSDK(World strongest Dai Kego) is the true translation then Mihawk is only the strongest amongst the Great Swordsmasters/Dai Kengos in the world.