It would be even better if it could accept numbers typed from a Japanese keyboard. This would make it the only good captcha I know that works in Japan. So please add this functionality, I really need it for my client, also.
Let me explain how you can do it:
You see the difference? The first one is Japanese. So when a Japanese person uses your captcha (the math question), and enters the answer in Japanese characters it will produce an error. So the user needs to switch to a western keyboard, but many don't know this. That makes the spam check unusable for now.
BUT, it is easily fixed. This is the UTF representation for １２３４５６７８９０, understandable by .php etc.: