It is not really a mistake at all if you take time to think about it. Kanto,Johto,Hoenn and Sinnoh are all a single nation. Yes they are regions that are separate but they are all in a single nation. While it has been said more then once Unova is another region in ANOTHER nation. So James saying Unite the people within the nation makes sense since it is not "his nation like the other 4 regions were in so he can't say "our"
That's a huge stretch.