I've got a new theory after watching the lates k-f-subbed episode of OP, and reading recent chapters of the manga...
First the facts:
There was once a group of archeologists who set out to find the phoneglyphs in search of the true history... Robin's mother was one of them.
20 years ago, their home island was destroyed by the bustier call.
20 years ago, the pirate king (Rogers) was executed.
Rogers also knows how to read the phoneglyphs.
Robin's mother and the other archeologists were either:
1: among the crew of the pirate king (or he was maybe also an archeologist, you decide), thus he also learned to decipher (read) the phoneglyps.
or 2: they met Rogers and got help from him to find the phoneglyphs, in exchange, they learned him to decipher them.
they're all togheter when the world government finally catches Rogers, and all of them are sent to prison, waiting for the execution.
Then Robin's mother got away and returned to her home island.
The world government are afraid of her and the rest of the world finding out about what happend during the blank age/years (??) or forgotten history, and sent someone out to stop her and everybody with the skill and knowledge of deciphering this.
My point being that there's a connection between the pirate king, the destruction of Ohara and the archeologists, skypeia (spelling?) and pretty much the entire history of the manga, all tieing up, revealed throughout the rest of the manga/anime.