I haven't had a chance to dig into this. I'm sorry, but I've been really busy with work and FPW is a free plugin that I provide to users without any warranty.
Debugging the compatibility issues with WPML isn't something that I can easily tackle since it will require actually setting up WPML, learning how it works and then debugging what may or may not be causing issues.
For clarification, are you trying to check categories from several languages (English & Vietnamese) in the same widget? I'm not sure if that will be possible since WPML only gives access to the selected language's categories when using that language. To FPW, English categories don't exist when you've selected Vietnamese or vice-versa.
I'll keep this ticket open and down the road I'll try to take a look at it again, but for now, I just can't give it the amount of time needed to thoroughly investigate. Sorry!