Copyright violation. If you write a book and I translate it to a different language you own the copyright on my translation. (except poetry which is artistic enough that it cannot be translated and so your version inspires me but I can't just translate it ). Decompilation doesn't have enough creative work to call it anything other than a translation.

I'm not a lawyer. I'm reasonably sure I'm right so the above is good enough for discussion, but if you need legal advice see a lawyer.

Actually you should look up the info there, you actually don’t which is what a lot of fansubs rely on, they mostly only will own your translation if they chose to formally translate and publish commercially a translation in that country. If they don’t, you can distribute your translation for free. There is a lot of variability on this per country too, with very interesting laws in greece and germany in particular.