Translation:December in Australia is summer.
This should also be correct. This software is way too flawed. There are way too many examples of where the translations are way to narrow and actually not written in a native or natural way, but in an almost literal translation. There are also too many high-context phrases where it could be I/he/she/we and one needs to just guess which it could be.
For learning Chinese a literal translation is way more important than perfect sounding english.
That should work if it doesn't.
I reworded as well to "Summer is in December in Australia." since it sounds more natural, but my answer was rejected.
"In Australia, summer is in December." would probably be the most natural, but I avoid trying subordinate clauses like that since they seem to invariably be rejected.
Edit: They do accept "In Australia, December is summer ." based on a suggested answer. (Yes, there's intentionally a space in front of the period character.) "In Australia, summer is in December." is rejected.
Nobody says "fruit is an apple" or "December is in Christmas". The logic in those sentences is wrong. You must consider what noun is contained in the other and not invert the order.
"Summer is in December" is wrong simply because it implies that summer consists of only the month of December. It does not.