In Itch, the only way to contact the developer is through the comments or the game forum.
It all depends, really. I’d rather make my offer known in advance, as it is not guaranteed that he or she would accept the translation in the first place. If they accept, they might give you the source or some other means, with which you would add the translation. The game might have quirks that ought to be known to translators, too.
Reverse engineering the game without notice might be seen as offensive. I, at least, would not take that lightly.