I agree this sounds odd in English. I understand the literal/direct translation, in this case I would suggest changing the original Irish sentence to make a better fit, or add a note in the correct solution to accept "I don't eat oranges" while specifying it is literally "I don't eat orange" in the singular form.
Does Ní ithim oráiste convey a negative habitual meaning in Irish? The English I don't eat an orange could be applied to something like a narrative: I enter the room, and everywhere I look is just… people eating oranges. Knowing what it'll do to me, I remain steadfast and resolute. I do not eat an orange.
This could answer the question "Don't you eat an orange every morning?" There is no rule that a habitual action must involve the plural, although we often do use the plural. Generalizations can use the singular. "An apple a day keeps the doctor away."
The next sentence after "I do not eat an orange." could be "I eat two." or "I eat a grapefruit." I would not take this sentence to mean that I never eat oranges, but as a particular repetitive situation with a time frame or meal that may have previously been mentioned. The singular can work with a fruit when you eat one regularly, but then again it is in the negative so you could be negating the quantity.
Present progressive is represented differently in Irish. The plural "oranges" would be " oráistí " rather than the singular orange which is " oráiste " http://www.teanglann.ie/en/gram/or%C3%A1iste http://www.nualeargais.ie/gnag/gram.htm (Click on "the Verb", then "tense and mood" and then "progressive" to see that tense in Irish.)
I can't think of any examples where you'd use habitual action with a countable noun without using the plural, unless you qualified it (e.g. with "every day" or by context). As a stand-alone sentence, it's weird. It's difficult to think, "this might be part of a huge conversation that goes like this... so it can work sometimes".