At first I was thinking, no it wouldn't be valid. I doubt illegal, but just not valid. Yu would have a hard time holding someone to something they didn't understand. Bt then i was thinking, that in most, if not all, the burden is on the signee to understand what they are signing. My wife and I a in the process of buying our first house, and we just went through the contact the other day. It might as well have been in another language, but I made sure I understood all of it before signing. And if I hadn't, I. Sure it all still would have been binding.
So, if yu give someone a contract, and they don't know the language, but they sign it anyways without knowing what it means, then I think that is their fault, and it would still be valid. But where things get tricky is with some form of translation happening, especially if you're the one doing it. If it were translated, valley by someone else, you have no idea what they said or how accurate it was, and it opens all sorts of doors for misinformation and misrepresentation. Or also, if you have basic skills in that language, you would be tempted to just paraphrase what it all means (or at least i would), but I think that would have the same problems. If the signee later has a disagreement with the way images were being used or something, it would be easy to make a case that what she signed was misrepresented.