Thank you guys for all your input.
I was going to ask the authors in any case, as I feel that it's what I should do before anything else, but also there's the matter of "i have permission to do this yet it may not be a very fond practice", looking at the romhacking community in general, and that's why I wanted to ask here too.
After all, I don't care if I can't put my name in this, I just want to bring one game into another language for those that can't speak english (Which sadly there are...quite a number of them in Spain. They should learn, though). But of course at this moment it's all but an intention, i'll see how I end up doing it.
About being true to the original text, fortunately I could check it whenever it's needed. I know how important is being faithful for most translators.