I'm gonna go out on a limb and assume you're a native English speaker, because even today, there are an awful lot of multilingual people in the world, with probably over half knowing at least 2 languages. The dominance of the English language worldwide has led to a lot of anglophones not only not learning a second language, but resultantly not developing the skills for picking up languages. It's pretty well known that picking up new languages gets easier the more languages you already know, which actually means that ancient diplomats probably had it easier. You see, nowadays we generally have this idea that languages are independent and relatively rigid things, but before modern communications technology local dialects sprang up quite easily, and they could vary a huge amount even within the same language. This means that anyone who travelled around in the pre-modern world would essentially have to be able to integrate different languages into their knowledge base much more regularly than someone would have to today. The nature of dialects also means that languages in contact would often develop pidgins that could be used as a way to learn the other language by people who knew one. The standardization of languages really put an end to that.