Theoretically, all of it, but the question is, how long can the Europeans hold on to it, and what is the nature of these colonies. Settler colonies are out of the question, as India showed, but simply administering an entirely native population isn't out of the question. Of course, the problem is going to be the willpower and resources of the Europeans, which is a tad hard to come by. The only people who could feasibly hold large sections of China for any extended period of time are the British and Russians, and even they didn't have much of a stomach to absorb the entirety of it. And then there's the Japanese, who will be genuinely jittered at a Scramble for China, but at the same time, they're by far in the best position to colonize large parts of China, if only because they're nearby.
Ultimately, though, I can't see any foreign power being able to hold significant parts of China for more than half a century. It'll be a massive drain of resources to bring it up to speed enough to exploit its labour and natural resources. And given how much resentment they've already whipped up just by manhandling the Qing into letting them run amok in their empire, I feel they're going to have a hard time on this one. The coastal ports they took IOTL was really the most realistic option, and Hong Kong and Macau did manage to stay in European hands until the tail end of the 20th Century.
It's unlikely for Europeans to emphasize local dialects, however, it is absolutely going to try to use English/French etc for administrative purposes. If for no other reason than to allow w/e Chinese officials they co-opt to communicate with their colonial rulers. It is unlikely to filter down to the general population until whenever widespread public eduacation becomes available after the colonizers had being defeated though.
English and French colonies both have a legacy of English/French language. India is the obvious example. I would't be surprised if English becomes a sort of semi-official language in post-colonial China simply because a large portion of the political elite will understand it. And nationalist individuals rebelling against colonial rule ironically enough are often segments of the westernized political elites.
Again India is the obvious example: Ghandi went to King's college, Nehru went to Trinity college. English is an official language in india today. It is very likely which ever Chinese nationalist movement will be led by some western eduacated segment of the political elites.
Um, a post-colonial nation keeping the language of the colonial power isn't a given, in all honesty. This usually happens when the colony in question feels that a local language isn't feasible enough to tie the new nation together, much less a settler colony where most people use the colonial language as a first language anyway. The Arab states from Morocco to Iraq all ditched it outright, for example. Even Malaysia and Indonesia, despite having a heterogeneous population, pretty much went straight into advocating Malay (Bahasa Melayu and Bahasa Indonesia) for
everyone, rather than English (to an extent) and Dutch (abandoned outright). In China's case, unless we're looking at a frontier region like Yunnan or Tibet, where non-Chinese speakers form the majority or plurality, the local dialect will probably dominate, with Mandarin as a prestige language and language of intercommunication between dialect groups. That English and Portuguese only really have a presence in Hong Kong and Macau among political elites and Eurasians (with English having a better grip, as the de facto global lingua franca), so I can't see any colonial language being able to supersede Chinese in the long run in a Scrambled China, unless tremendous effort is taken to settle populations there, a ridiculous feat to pull off against the Chinese, even for the Russians or Japanese.
