Interesting point. I'd say one language because I think quite a bit of the worlds population were affected by the beasts and so the remaining countries all took a united stand. Over time as they together their cultures blended together and they would decide to adopt a common language. You see it in the myths about the moon with Greek and Chinese myths mixed together. I would assume language like culture also mixed together into one. As for the china part, they could have been the predominant population left and assimilated other cultures.