[hijiri]

The GPL does say the conditions are only on redistribution. But I think that's how it works with other derived works as well, by default. The GPL is all about how you can relicense it to other people using only the same license, you're free to do whatever you want if you never give anyone your modified code.

[schoen]

17 USC §106 refers to the right "to prepare derivative works" as an exclusive right of a copyright holder, so creating the derivative work in the first place could be a copyright infringement under U.S. copyright law even if it is never distributed to anyone. So you can potentially infringe copyright in your basement by making a fan translation of some movie or novel into your language.