Jake is US-American with Czech origins, that is why he has a difficult time telling Chinese and Japanese apart. They all do look the same, apparently. To the casual western eye.
Of course, coupling German with Japanese was handy due to the Axis Powers connection, but he needn't have gone that far: China was an (earlier) ally and supported by the Reich against the Japanese invaders all during the 20ies and 30ies.
That only ended when the Japanese explicitly asked for it to stop in late 1941 following their little Hawaiian surprise visit. Just look at Chinese uniforms of the time ... (and the Kuomintang National Revolutionary Army was trained by German generals at the time too):
