When I hover over "an orange", one of the translations it shows is "en oransje", yet it doesn't accept that answer and wants me to use appelsin instead. Is that just a bug or is there a reason it should be appelsin instead of oransje?
Appelsin is a male word, so it is always "en appelsin", no matter who is eating it. The sentence would be "Hun spiser en appelsin" with a girl.
You'll just have to get used to the genders, there is no workaround.