In spite of the problem of language, it was clearly not absolute. There was some way to communicate, some lingua franca that allowed for at least some understanding. The communication was increased by the teaching of the Nephite language. This was essential to assure that the records would be able to be read by later generations, and kept. It is unknown what these languages were, although there is evidence in many names, which suggest that there was an ability to write in Hebrew to preserve those meanings for our times.
It is significant that the plates are again mentioned prior to noting that the peoples were united. It is very likely that the possession of the plates was the fulcrum on which the Nephites were lifted as rulers over the people of Zarahemla. We remember that verse 17 noted that the people of Zarahemla were very numerous. While the Nephites would also have been numerous, they had fled their city and probably left many behind. Thus, it was a refugee population that merged with the more numerous people of Zarahemla and that accepted Mosiah1 as the king over the united peoples.