I think they addressed the issue of a basically Japanese crew due to the condition the planet it in at this stage of the war. The surviving regions of the United Nations are more or less isolated from each other which would make training more difficult. It also might be that their best and brightest have already been killed in the war as the larger counties would have been targeted first.
It also might have been luck of the draw. Defense Command might be in Japanese hand by a matter of rotation. Or that was just where the first Iscandarian ship landed, so processing from Japan was the quickest solution.
The First Fleet was entirely made up of Japanese ships. Likely because the other fleets had been destroyed prior to this last engagement at Pluto. And the background notes seem to suggest these were older ships to begin with. Kirishima
and the cruisers are supposedly built in the 2270s so they are 25 or so year old ships going up against an advanced alien aggressor.
That they managed to build this giant spaceship under what looked like the wreckage of an old battleship was done specifically to keep the Gamilas from knowing what Earth was doing. Which means the humans know the Gamilas have fairly good recon abilities.
And a part of the orignal officers canditates were killed a day or two before the ship was to launch. They had to rush to get replacements from the local pool...that being Japan. This is alway why you have several people getting heavy promotions at when they board Yamato
. (Shima, Kodai, and Yuki are Ensigns at the start, and by the time Yamato launches they are full Lieutenants and section leaders. Nanbu gets a one grade promotion to Lt. jg and head of the gunnery staff.) Considering that Kodai's bother was suppose to be the Combat Chief and he was a Lt. Commander with his own commad...I image he was suppose to be the ship's orignal XO instead of Sanada.
But there are other survivors other than just Japan. Judging by amount of power provided, the Americans seem to have the largest remaining powergrid followed by Europe and the Japan.
One thing to note. The show's General Director, Yutaka Izubuchi, was involved with the mechanical art production on Yamato III
. He designed the Space Battleship Arizona
, which was specifically an American built ship in the series. It looks like a cross between Yamato
in style and has more guns than both.