"a liter of juice"
The English is one litre. We work with two languages and if one speaks about one litre or a litre it remains 1000ml of juice. A litre is a singular litre of juice, therefore one litre. Finnish is difficult enough without extra catches. Yksi litra mehua is a litre of juice.
The English is a liter. It's not a question of whether there is 1000 ml of juice. Of course there is. The point is that the yksi is not said in Finnish unless there's a reason for it. In the same way a and one are not used completely interchangeably in English. Therefore:
a/the liter = litra
one liter = yksi litra
It's no more complicated than that. Translate the number if it's present. Otherwise not.