If I download a wordpress theme that is covered by such license and then translate it to another language, can I encrypt part of the footer of the php code so users won't delete the "Translated by ..." part on it? I don't think it's fair that they delete the footer link while I've been spending hours on that theme.
I'm not a GPL export, however, probably not. The purpose behind the GPL is that the source is available free of charge, so you would need to make it available more than likely. While it's not fair, you're also having the benefit of a free theme - so is that fair for the original author?
You would be in violation of the GPL license by doing that. The whole purpose is to give developers the freedom to customise the software, as you've done. Encrypting portions of it undermines this philosophy.