It's assumed they were other Seed Ships from other countries, the American one just happened to crash on an alien world, but I wouldn't be surprised if other nation ships play a part in the plot. Technically I bet the Earth does have a Federation of some sorts (thus the cast will probably be of mixed background) and we're just following the misadventures of the Neo Los Angeles ship which probably has people from all over Earth on it. It will be interesting to see how the story pans out.