Since the game is quite heavily localised you will feel frustrated at times trying to communicate with other English speakers when the words are different and they don't understand or accommodate this (I played FFXI in full Japanese for ten years and JP players were easy to understand while players with other clients would be confusing due to auto translate being used for the wrong things; the words seldom made sense across clients). There are also small things like titles being different (yet still in English) which might spoil some of your favourites. Since it's easy to switch though, why not give it a try?