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 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.
Earlier DL states "mafuta inachomeka" means "the oil is flammable", but now I get marked incorrect for translating "majanai mabichi hayachomeki" as the "green leaves are not flammable".
Why is this not acceptable? It portrays the same understanding! I noticed with Swahili one has to literally state verbatim what the language translation has been programmed as. Duolingo Team, please look into this