The 21 Earth ships at the Battle at Pluto were all Japanese ships with Japanese names. However they were all old ships by this point in the war (the battleship and cruisers were from the 2170s). It is stated that this was all Earth had left. If all they have left is nearly 30 year old Japanese warships after fighting the Gamilas for 8 years, that means everyone else's fleets have been destroyed and Earth's last gasp is basically a self defense fleet from a second or third line navy.
There is backstory about another space war between Earth and Mars. One assumes that is were the UN forces using different nations ships comes from. We know the Earth (or Mars) had a mining base on a moon of Saturn, so by the late 2100s humans have expanded into the solar system quite deeply. The Gamilas attacking would unite humanity against a common foe, but the Gamilas have major techological advantages. The Earth may have had numerical advantages early in the war, but by Pluto, the Earth Fleet was outnumbered at least 6 to 1. The only victory that we've been told about in Operation K-2 (the Second Battle at Mars) under Admiral Okita (the hero of the Second Battle at Mars). Yet we don't know a lot about that save that it was a costly victory. The Earth fleet turned away the Gamilas advance. The Earth ships uses their new bow mounted Shock Cannons to do damage to the Gamilas ships. The results was the stepping up of the long range planet bombing siege of Earth, instead of a direct attack or invasion.
The war pretty much equalizes the surviving nations of Earth it seems. With Japan, under the Far East Regional Command, having the only two known warships left (Kirishima
) they have a leg up when Earth recovers. But the UN is still in charge, and judging by the power grids the EU and United States still have the largest amount of reserves, though I image they were targeted first by the Gamilas to remove their space forces early.
By 2201, if things go as it did before, the Flagship Andromeda
could be anyone's design, but the most experiance surviving flag officer is Hijikata. The new Earth fleet could have been built by the Americans for all we know.