1. Chinese version only works on China server.
2. -
3. All EU sensors that I tried, work on china hub/server and eu hub/server. Sensors that you can only buy in china only work on china server/gateway. For example, chinese window/door sensors also work on the eu-hub because they are the same that you can buy in EU. I think, all sensors that you can buy also here, work with the china hub.
Advantage China hub: every sensor (no matter if EU or China) seems to work
Disadvantage China hub: only works on chinese server. The problem there is, that it is VERY slowly when you are not in China because of the big firewall. Also Google push notifications don't work reliable. Sometimes I get a notification 20 minutes after the event, sometimes instantly.
Advantage EU hub: Fast EU server, instant notifications, everything works as it should be
Disadvantage EU hub: Only EU sensors work, so you have a VERY small choice. No wall switches, no fire sensors, no relays, etc. Not really a smart home.
Conclusion: At the moment, I would not buy a Mi smart home again (no matter if chinese or EU). But when they release more sensors here, it will be a good choice to buy the EU hub.