I think Yugoslavia could be studied here. Serbo-croation as a sort of official language along with the languages of all nationalities in the federation is a probably a good basis for a form of international language. Perhaps something more along the lines of conglomerates of a language group used between speakers of languages in the same group, with a certain international language taught to everyone. However I do think the third language is a bit of a reach, (being trilingual-ish), but two languages could be managable for most people.