I think that - fresh ( can be also green ) leaves can't be burnt - is working better :)
In my 58 years of life I have never heard anyone say in any variety of English that something is "unburnable"!!!! Yes something cannot be burnt, is not combustable, is flame resistant......
I'm not a native speaker, but I think that 'nonburnable' is correct English, and hence an good translation.
I am a native English speaker and I would never say "unburnable". I can see what it might mean, but here I would probably say "Green leaves don't burn well."
I am a native English speaker and would say unburnable, but I'm from Wisconsin and we tend to put un- on lots of things. My coworkers give me grief because I "unpeel my bananas" . Google Manitowoc Minute Unthaw for another example.
unburnable and not burnable are the same thing
Did you report it?
How do you manage to learn all those languages?
Why doesn't it accept fresh leaves for green leaves?