nothing,,just that you can not use forum app, some chinese system apps will be there which can not be uninstalled or disabled, you can disable data access to avoid unwanted chinese notification from those apps,,and some apps are in chinese and do not support English like weather, browser, app store, themes etc

what posibly go wrong of i use this Chinese version of flyme on my global version M2 note?

yes..

doe it support english language?

http://www.flyme.cn/firmwarelist-10.html#3

Chinese beta....for m2note