I understand that One Piece is anime and not real, but come to think of it, how come everyone speaks the same language. In the Punk Hazard Arc, Kin'emon exclaimed about foreign lands from where the straw hats came. But still, everyone with no exclusions speaks the same. Any indications or anything?
From the evidence that's currently present, it seems like this is a side-effect of the world having a single government.
For instance the phoneglyphs aren't in Japanese, since only Robn can read them. If they were a simple alphabet change, then someone would have figured it out already. This means, that there were different languages back then (at least one).
So, there's a chance the world was as diverse in terms of language as ours, but then the void century ended when the current world government defeated the others. Just think about it, if Japan would take over the whole world, then everyone would speak Japanese here too.
The bigger question is, that why didn't they develop accents. The islands make the people isolated into small groups. This is a perfect enviroment for that to happen. Even half a century should be enough to result in at least some small changes.
This was answered by Oda in SBS Vol. 25:
Reader: I have a serious question for Oda-sensei. How come, in manga, no matter where you go, everyone always speaks the same language?
Oda: Because manga is something where everyone's dreams are depicted.
Besides this, I remember another statement from Oda where he said the offical language in the series is english (no source, sorry). This is indicated when you see an english name for an attack or a character. (e. g. "Nations Might" is not written in kanji in the manga.)
It will be hard if every civilization has its own language, not just in different languages but also in translating the manga and anime dub. They should always put subtitles and have translators in anime and manga like, for example, when Straw Hat crews meet an animal, Chopper starts translating to them what he's saying.