Translation:I need to slice potatoes for the soup.
This is complicated, because in Russian, some foods are always regarded as singular (mass nouns) and others are treated as usual nouns and decline as either singular or plural. We unfortunately just have to memorize which is which. In the case of картошка, it always declines like a singular noun, whether you are talking about one or more than one potato. Consequently, I believe "potato" should be accepted as an answer as well as potatoes, since we do not know from the question whether the the cook will cut one potato or more than one.
Below is an elaboration with examples of food nouns used differently from a website. The final example below is like картошка:
In Russian, we use some fruit and vegetable names as mass nouns and don't form plural for them:
Салат с луком / капустой / клубникой while the others do form plural:
Салат с огурцами / кабачками / артишоками / помидорами Also, if you ask someone:
Что у тебя в сумке? then the answer:
У меня в сумке огурец / кабачок / баклажан / артишок would unequivocally mean there's exactly one vegetable in the bag, while this answer:
У меня в сумке лук / капуста / клубника may mean any quantity.
There is a similar usage in English, where we use the singular form to refer to an unspecified mass amount of something. You can see it in the salad example: A salad can contain carrot, tomato, onion, etc. - without specifying the quantity. Even a huge salad prepared for thousands of people in multiple bowls - obviously requiring more than one of each vegetable - could still use the singular form of nouns, although plural would be OK too. There's no strict rule of English grammar on this.
I copied all the Russian text at one time and pasted it all into Google Translate, and it gave me the following as a translation of DavidG430 s examples (with some rearrangement, but no changes to the words):
Салат с луком / капустой / клубникой
Salad with onions / cabbage / strawberries
while the others do form plural:
Салат с огурцами / кабачками / артишоками / помидорами
Salad with cucumber / zucchini / artichokes / tomatoes
Also, if you ask someone:
Что у тебя в сумке?
What's in your bag?
then the answer:
У меня в сумке огурец / кабачок / баклажан / артишок
In my bag, a cucumber / zucchini / eggplant / artichoke
would unequivocally mean there's exactly one vegetable in the bag,
while this answer:
У меня в сумке лук / капуста / клубника
I have in my bag onions / cabbage / strawberries
may mean any quantity.
You are right. Most of "food nouns" in russian are mass nouns, like картошка/ картофель ("картофелина" is a singular noun), морковка ("carrot". this one also a normal noun, but used as a mass one), лук (onion) ("луковица" is a singular noun, if you talk about underground part of the onion. "The green" part is a mass noun)
No, it's just that "картофель" is considered to be a more "proper" word, while "картошка" is a bit more colloquial, so that's why Google translate defaults to the former. However the quantity of potatoes makes no difference as "картофель" is a mass noun too.
If you really want to count potatoes, there's the word "картофелина". "I need to slice three potatoes" - "Мне нужно нарезать три картофелины".