The point is we speak English, so it's referendums, agendas and paninis.
But also indices, larvae, algae, analyses, crises, theses, matrices, addenda, bacteria, millennia, strata, stimuli, syllabi, etc etc
Agenda is the plural of agendum so agendas is wrong. Mind you, technically agenda is incorrect if used as a singular noun but the convention is now to use agenda for the singular.
It depends on how it's borrowed from the original language and how it has evolved.