"Hunger is one of the most serious problems that is."
Translation:La faim est l'un des plus sérieux problèmes qui soient.
Either the English sentence has broken grammar, or some crucial context is missing and the translation is wrong.
Option 1: The translation is correct
If the translation is correct, then "that is" refers to "the most serious problems". But then the number is wrong and "that is" should be plural as in the French version:
"Hunger is one of the most serious problems that there are [or: that exist]."
Option 2: The English sentence is correct
Since "that is" cannot refer to "the most serious problems", it must refer to "one": "Hunger is one ... that is." This makes sense if the preceding sentence sets an appropriate context: "Many of the world's ... problems are not solvable. Hunger is one ... that is." Now we just have to fill in the dots:
"Many of the world's small and big problems are not solvable. Hunger is one of the most serious problems that is [solvable]."
But this is irreconcilable with the French translation. If "qui soient" referred to "l'un", it would have to be singular indicative, not plural subjunctive.
This is one of the silly little details of French grammar that just have to be learned. IMO it doesn't really make sense. I just checked "est un des" and "l'un des" as well as "est une des" and "l'une des" in the real French literature using Google's ngram viewer. (I would give direct links, but it's very cumbersome and the forum software eats the necessary ampersand characters.) I was surprised by what I found.
The good news:
The superfluous article doesn't always appear, but only in about 50 % of uses in the masculine case and just over 40 % of uses in the feminine case.
The bad news:
It's on the rise. In 1800 the definite article was only rarely used in this way (less than 10 % of cases). But since about 1920-1940 there has been growing use of the definite article. At the moment we have roughly parity, but there is no indication that the process has stopped yet. Not using the definite article here may soon be considered wrong or at least unidiomatic.
For some reason that eludes me, it appears that using the definite article with un has always been slightly more popular than using it with une. I would be very grateful for a plausible explanation for that.
But problème is masculine, and this is what matters. This is for a similar reason as why the English sentence is actually ungrammatical. (See my previous post for the explanation of that.)
In each of the following examples I will use parentheses for one phrase that forms a unit which needs to be analysed together and can be replaced by a single word. In the next line I will repeat the sentence with this single word, ask for it and give the response.
The correct analysis is as follows:
- La faim est (l'un des plus sérieux problèmes qui soient).
- La faim est quelquechose. Elle est quoi ? L'un des plus sérieux problèmes qui soient.
- Hunger is (one of the most serious problems that there are).
- Hunger is something. What is it? One of the most serious problems that there are.
Your analysis is plausible, especially coming from English. But it is not the one actually used by French speakers. Here it is in detail:
- La faim est l'une (des plus sérieux problèmes qui soient).
- La faim est l'une. Elle est l'une de quoi ? Des plus sérieux problèmes qui soient.
- Hunger is one (of the most serious problems that there are).
- Hunger is one. It is one of what? Of the most serious problems that there are.
Finally, for comparison the nonsensical analysis used by whoever wrote the wrong English translation:
- La faim est (l'un des plus sérieux problèmes) qui soit.
- La faim est quelquechose qui soit. Parmi toutes les choses qui soient, elle est laquelle ? L'un des plus sérieux problèmes.
- Hunger is (one of the most serious problems) that there is.
- Hunger is something that there is. Of all the things that there are, which one is it? One of the most serious problems.