It's probably not legal but it's in the "no one really cares territory" since to actually enjoy the translation, you need to either buy a copy of the game or pirate it. In the former case, great, a sale made to a person that would've never otherwise bought the game and in the latter it makes no real difference since you were never marketing to that person anyways.

The English release of Steins;Gate is actually an edited version of the fan translation, so the property owners couldn't have been that upset with the fan translators.