Ironically the odd 'pirate speak' found in Limsa (especially with the members of the ROG's Guild like Jacke) makes perfect sense - it's
Thieves' Cant, supposedly a subsitution language used by pickpockets, pirates, criminal gangs and subversives hundreds of years ago, although again like the Mock Shakesperean 'ye olde English', it's more commonly found nowadays in fantasy and historical fiction stories to give a kind of mystery and suspense to characters.
Of course, the other side of the coin is that it makes it so hard to understand
just what the characters are saying, especially if you're unfamiliar with the concept and think it's just nonsense (which I suppose was the whole point of it in the real world). Either way, it's use in FFXIV makes perfect sense in the setting, and is meant to be yet another part of worldbuilding, establishing the setting and tone to the player.