Language is certainly useful, though some--like Jerry Fodor--have posited that there is a separate, internal language of thought (mentalese) that is distinct from our public, shared language. It's possible that our creative acts rely on this internal representation. I don't think we have any idea of whether other animals have their own internal representations or what they might be like. Does a gazelle fantasize about having wings so it can escape the lion? I rather doubt it, but I can't really be sure. I'm not entirely sure about this, but I suspect that counterfactuals--that is, things that don't exist and might even be impossible--might require language to be formulated. If that's true, then I think one can claim that language is necessary for creativity. OTOH, when I think about musical creativity, all this seems to break down unless you insist that music is a form of language, which seems like more a metaphor than a description. Dunno, but it's an interesting question.