Technically, it is illegal to modify code. Although, usually fan translations are aimed at non-locally copyrighted material, which means there is nothing the company can do.
Then there's as you say the issue of applying the patch. On console gaming, there's pretty much no way for it to be legal. On PC gaming, with the game (as opposed to the ROM), the only offense is the one mentioned in the preceding paragraph.
So given that it ranges from grey to illegal, I'm surprised at Eusis' phrasing in his edit, given how strict the rules are here. (and since I feel like somebody may comment on that last bit, I'm fully okay with the rules, their forum, their rules. Simple.)