Although Duo teaches US English we make every effort to include all other versions. For this sentence we have...This is my sweet. This is my candy. This is my dessert. This is my lolly. (the last one suggested by another Aussie).
If there is anything else you can see that we need to add please let us know.
However, when it comes to which words are presented to pick from it's the computer algorithm that determines the selection.
Well, I'm a US-English native speaker, and I have to admit Alex has a point. This is my dessert. These are my sweets. (plural noun). Sweet is the adjective.
To me, "this is my sweet" would be a way of introducing the love of my life, not a reference to food. At the very least, the singular noun has this as an additional use.
However, I tend to use language rather precisely. That's becoming increasingly uncommon in the US.
The main translation here for American usage (which we teach on Duo) should be "This is my dessert." However, since we also try to cater to other forms of English somehow "sweet" was entered and we do not want to confuse the many learners who have already seen it by chnaging it in mid stream. That will be done in the new tree under construction. We do also accept as correct: sweet/candy/dessert/pudding/lolly the latter for Australia.
Thanks for your comments and keep them coming.