It probably depends on what you mean by "theoretically possible."
For chimpanzees, I think it highly unlikely that they would maintain language after release into the wild. Apparently the bonobos do better at language than chimps, so there is more of a possibility that bonobos might retain language.
Humans appear to be built (i.e. to have evolved) to acquire language in early childhood. If you were to start a colony without a language, the children in that colony would soon invent their own language. However, chimps and bonobos won't do that. So a reasonable conclusion is that they have less evolved support for language than humans.
If a group of bonobos were trained in sign language, and were trained in such a way that they were able to use their sign language to communicate with one another and improve their lives by means of this communication, then perhaps they could retain it when released in the wild. But if their life in the wild finds no particular benefit in the use of sign language, then that language would probably die out.
At least that's my best judgement. I could be mistaken. It might be an interesting experiment, if it can be done humanely.
In the meantime, here are a couple of links to reports on the use of language by apes.
Primate Use of LanguagePage has gone | New Scientist