In its long history, China (and any number of other empires) has conquered and assimilated ethnicities with different cultures, languages and skin colors. East Turkistan, Tibet and Mongolia all speak a language very distinct from Chinese, and much closer to Japanese. Their lands comprise 50% of today's Chinese empire. The vast majority of today's "Han" Chinese population were barbarian peoples subjugated and assimilated over thousands of years. The founder of China - who cut the population of China in half during the unification process - was the leader of a barbarian kingdom. The bottom line is again that victory means never having to say you're sorry.
Before the unification of China, there was no China. After the unification of China, the perception is that there has always been a China. The reality is that while the land mass on which China rests has always existed, the political entity that is China and the ethnicities that reside within it are the product of armed conquest and forced assimilation enforced with occasional exemplary large scale massacres like every other empire. A Japanese victory would not have significantly changed the biological facts, but would probably have displaced, at minimum, the Chinese language, much as the first "Chinese" emperor proscribed the use of written languages that preceded his rule (much as Turkey has attempted to stamp out the Kurdish language).
Well, no. East Turkistan/Uighuristan speaks Turkic, which can be related to Mongol, but in Tibet the Tibetan language is part of the larger Sino-Tibetan-Burmese language
Japanese is a language isolate, supposedly in the Altaic family related to Nenki etc. in Siberia but not proven.
Also, the statement of the unification of China is wrong -- the Older Zhou dynasty or the Song dates to 2000 BC and that is the origin. The other peoples got assimilated, the latest being the Manchu/Jurchen
But the unification of China didn't occur - this was reunification