You have too narrow a perspective.
China by itself is a lot larger than all of Europe. Roughly speaking, China has twice the population and land area.
And if you look 2000 years ago, China is composed of what were many pre-existing tribes, just like in Europe.
Yet now, China now has a single national language and a national identity.
They do get along with each other, like your German-Italian example.
In contrast, we can see an ongoing fragmentation of Europe with a resurgence of Tribalism and Nationalism.
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You want to compare relations between China-Korea and say these countries "don't get along"
Well, the European equivalent would be Europe-Russia or Europe-Middle East.
Russia relations are awful, given there is an ongoing military war.
And we see a lot of anti-Islamic sentiment against immigrants from the Middle East.
