Well its deffinitly a lot easier to flash all three at the same time, but technically all you need is to just flash the main with either like you said CHINA_MAIN or TAIWAN_MAIN or HONG_KONG_MAIN and thats it. Then you would just need to place the .lng and .t9 chinese files onto your phone and edit the customize.xml. I wrote a whole tutorial on how to do it here how to add languages to your phone - Top Sony Forums you can then also do it that way to add your french language as well. but like i said, you will deffinitly need the CHINA_MAIN for the chinese language and it doesn't matter at all the main for the other languages.