>However, forcing the invader's culture and practises on the conquered people and taking steps to replace the local language, culture, politico-legal systems and social structures is.

That's orthogonal, more like cultural imperialism. Some colonizers did it, others were just fine with not imposing any of their culture on the locals, simply because they merely wanted to use them as slave labour or primitives to be utilized.

It's also not an exclusive sign of colonialism. All countries that emerged from a merger or combination (like in Europe, from Spain, France, Italy, and Germany, to Austro-Hungary and the balkans), whether through war/annexation, peaceful merging, or revolutionary nation building, unite the culture of the parts, usually based on the dominant cultural group from before. This happens both for business and governing needs, but also organically through job migration, commerce, and so on - and often includes some coercion. Not sure China's case is heavier than others, especially given the era.

It's always been strange to me that it's seen as a worse crime if the guy who takes over your village is a European who arrived on a boat rather than the neighboring warlord who arrived on a horse.

>Ask, e.g. the Baltic nations whether what Stalin did to them was not "colonialism".

I could ask them, but I would get diverging opinions, especially if I go back to the generations that actually lived under that (and who often expressed how they find it "better" in several ways, in published post-1991 polls).

Not to mention the civil tension even at the start of this, pre- and post-WWII, between pro-communists and pro-USSR at the time (which had sizable popular support among European peoples, from Italians, Spanish, Greeks, Portuguese and so on, including sizable numbers in France and Germany) and also much more remote parts of the world, and which was squashed with quite a lot of blood and far right terrorism. Czechoslovakia itself had quite a strong communist party, way before 1948.