See, considering that this is a net positive thing, and does not in any way harm or deface the original product, that's why I was asking. To my way of thinking, this would not be something I would think to ask for. I'd give a heads-up instead, with the caveat that if the dev wanted to work with me, or make things easier, or had feedback, my door was wide open. Ditto if they had some particularly good reason why I shouldn't proceed. But it would be with an "unless you have an objection" angle.
That said, I fully respect the bit about Japanese culture. That is something I would not have known for myself, and I would like to think that I'm culturally sensitive, enough so that, knowing this, I would now make the appropriate gesture instead of perhaps upsetting or insulting the devs. Obviously, this wasn't on my plate in the first place, but I appreciate learning things like this.
If I ever make a game though, and someone wants to translate it into a language that isn't English, my general stance would be that you don't have to ask me, but you should try as hard as you can to stick to what's written. Don't change the dialogue from the original, and try very hard to do a good translation. If you want me to package your translation with my product, then we have to collaborate somehow and I'd have to make sure that your translation is professional and up to quality standards. Beyond that though, you'd get a huge thank-you from me even if you didn't ask first. You're bringing my product to a wider audience, doing work I cannot do, and that's huge.
Check out my Manamon text walkthrough at the following link:
https://www.dropbox.com/s/z8ls3rc3f4mkb … n.txt?dl=1