There were more than 10,000 years before the first war, and earth's location may or may not have been forgotten during that time. Earth's location is definitely forgotten now (all the star charts are gone, or outdated, during the sequestration)
As for dialog, both factions are a billion strong. They probably have several internal languages, and most if not all of them are probably new. That said, they can talk to each other if they want- the elites, at least, know the original trade languages.
In game terms, earth is unknown and might not appear in this game, and when the factions meet, there will be the potential of communication, though you probably will be mainly fighting.