During the summer event, players can obtain a watermelon slice by clearing a daily dungeon. These slices are used as tokens to unlock these outfits. The problem is that it can be difficult to actually obtain enough slices through play unless you focus on running dungeons with a single-minded devotion, which forces you toward the cash shop option, which values the suits at $45. Still no word on when the game is being localized, but you can check out the suit trailers past the cut for a peek at what you probably aren't dropping $45 on.
[Thanks to bnssswimwear for the tip!]