"Of the two girls, she is the youngest."
Translation:Van de twee meisjes is zij de jongste.
In this case, "het jongste" wouldn't make sense, but I see where you are coming from. For many adjectives, you can also use "het" + superlative (even for de-words), but the meaning is slightly different.
To illustrate this, let's use a different adjective:
- De boom is de grootste - The tree is the largest (of all the trees in the forest)
- De boom is het grootst(e) - The tree is largest (when it gets enough sunlight)
In the second sentence, the English translations doesn't have an article with the superlative. This is the case when we are comparing the size (or another quality, for different adjectives) of the same object in different situations or under different conditions. On the other hand, in the first sentence we are comparing the size of one tree with the sizes of the other trees.
In this sentence here, it is clear that we are comparing the ages of two different girls, so we must use "de jongste" (and the youngest in English).
I dont think you get the problem here.
Its about the noun "jongste" to refer, I see a lot of sentence with similar situation. And if the sentence is "jongen" there will be no problem
e.p. "Van de twee jongens is jij de jongste (jongen)" But here is "Van de twee meisjes is zij de jongste (meisje)" and "het meisje" it should be "Van de twee meisjes is zij het jongste (meisje)"
Just wanted to point out that, strictly speaking, you can't be the "youngest" (or the -est anything) of only two things. This rule is frequently ignored, however. Is it the same in Dutch?
In attributive use, Dutch uses the superlative form even when comparing two things.
So before the noun: Hij is mijn oudere broer.
But: Ik ben de oudste van de twee jongens.
Same same, but different. Yes you are saying the same but obviously the sentence is different. Personally I would stay as close to the sentence that needs to be translated, especially when doing so is perfectly valid.