On the big X10, if you go to compose a message and press and hold the area where you go to enter text, the phone will come up with options including 'Input Method'. Is this the same on the mini>? If so, if you select 'Input Method', does it come up with different options? For example, mine gives three options: 'Standard keyboard', 'Japanese keyboard' and 'Android keyboard'. If it is the same, select 'Standard keyboard' or 'Android keyboard' and you'll lose the Japanese characters.
If you mean that you've checked this and you really only have an option for Chinese, I don't know what to suggest. Phones for different regions come with different options, and I would guess that phones destined for China would have Chinese input and not a lot else. So, if you imported your phone, it might be that it has software that differsd from that usually available in your country.
I do hope you meant the first of the two scenarios - it's a lot easier to fix!
