The English version is less comprehensible than the Esperanto version.
By which language did the people there communicate?
it accepts: "By which language did the people communicate there"
That is part of the point of Esperanto.
The English version is awkwardly worded. "Through what language did those people there happen to communicate." Still not totally natural, but a lot better than the literal.
Why “those” people? Why “happen to communicate?” The Esperanto sentence says just “la homoj” (people) and “okazis komunikado” (did communication happen, or did … communicate, not did happen to communicate, which is a different thing).
Tiu frazo estas tre malbela.
This sentence is awkward in English, and much easier to understand in Esperanto. "By means of which" would not be used in English, we would probably just say "How did it happen that..."