In the past, diplomats did not actually represent nation-states, but royal courts (in most monarchies this is still officially the case). As a result, European diplomats, for example, were mostly aristocrats, who all spoke French to one another.
过去，外交官实际代表的并不是民族国家，而是皇家宫廷（多数君主制国家现在这种状况依然成立）。因此欧洲外交官大都是贵族出身，彼此交流时都讲法语。