I think that luffy can hear the poneglyphs, but probably he can't understand them, they "talk" in their own language. For that reason he needs Robin, and Roger needed Oden. When Momonosuke was hearing the poneglyph, he said that he was hearing a voice, but no that he can understand it.
Three eyed race probably can understand any language, so they don't need to learn the language.
Zunisha and Sea Kings probably talk in the same language as everyone, but only a few can hear them.
@Long:
Just realized this might have been forshadowing as well.
What was Duval's job before sanji got his bounty?
! [qimg]http://img.mangastream.to/manga/one-piece/495/AT_OnePiece_495_pg (3).jpg[/qimg]
Mafia
Bartolomeo too, and he didn't meet Sanji :whistling:
Not really related to the topic,just out of curiosity.. as most latin american countries speak spanish, this mexican translation could be used for the other countries (argentina, paraguay, venezuela, chile) or each one has drastic variations even in the same idiom?
One Piece manga is being published in Argentina by Larp Editores (the last volume is #25).
Regarding translations, here in Argentina (and also in Uruguay and Paraguay) we use "vos" instead of "tú" as the second-person familiar pronoun in every day speech (and, of course, "vos" conjugation is different). It's called "rioplatense spanish" and we never use "tú".
Both Larp and Ivrea (another argentine publisher) use "rioplatense spanish" for their translations, but we are so used to watch tv shows, movies, cartoons, anime, etc dubbed in neutral spanish (where "tú" is used) that some people find really distasteful that Luffy speaks in the "vos" form and prefer a more neutral spanish translation, so they don't buy the volumes. Honestly, I hate that people. 
Coby, but better Nami and Sanji. They form the perfect couple:
