They choose the language that they submit it in, and there are rules prohibiting people from having two petitions open simultaneously that are on the same topic. If you tried to enter the same petition in another language, that wouldn't be allowed because it's the same petition, so they're stuck submitting it in one language.
However, as I say, if a petitioner expresses an interest in verifying the translation or wants to have a say in what the translation looks like, we can certainly co-operate with them. There's an email address that they can send questions to, and we get back to them.