Finding a fixed-size set whose members are contained by the largest number of other sets
I believe your problem is a direct instance of the NP-hard Maximum Coverage Problem, which is related to Set Cover.

From wikipedia, Maximum Coverage Problem:


  As input you are given several sets and a number k. The sets may have some elements in common. You must select at most k  of these sets such that the maximum number of elements are covered, i.e. the union of the selected sets has maximal size.


So, in your case, there is a set for each language with cardinality equal to the number of students speaking that language. The input is the number N of maximum number of translations.