Why are they speaking English in this?

Because otherwise poor souls like you would be unutterably confused at the lack of English and wander off to twerk or text or twitter or something.

Well, this was a movie made by English speaking people.... and movies made by English speaking people tend to speak English... LOL. But seriously, I don't think that (if they HSD don't tgis in the appropriate tongue) that it would have been in Roman.... wouldn't it have been in Greek?

Why are you such a troll?

Why am I feeding you right now?

a. It's Greek mythlogy, not Roman.
b. Would you understand it in ancient Greek? Even with subtitles I suspect you'd be hard pressed to keep up.
c. There's no such language as Roman. The Romans spoke Latin.

Yeah very few movies use what should be the actual language spoken. 'Valkyrie' from a few years ago was set in Nazi Germany but I don't recall Tom Cruise speaking German throughout. Heck why do the main Star Wars characters speak English? That can't be right. In fact the only recent film that did use authentic languages that comes to mind is 'The Passion of the Christ' - wasn't that done in Latin, Greek, Hebrew and Aramaic? But that is the exception to the rule, those are few and far between.

It's so you can understand and don't walk out because you "go to the movies to watch movies, no to read".

Source: http://tvtropes.org/pmwiki/pmwiki.php/Main/TranslationConvention

Look, first of all, this was a film designed for English-speakers to watch, not ancient Greeks. Second, most people understand that when they're watching ancient people speak English, they use their imaginations and realize that the characters are speaking ancient Greek, but we hear English so we know what they're saying to each other.

It's a common understanding more intelligent movie audience members have when watching a film where it's pretty obvious the characters wouldn't speak English for real, but the actors are doing it for clarity.

Not to be an a ho but I appreciate the English part because it's the only language with which I am fluent.