Throughout history there have been many lingua francas that served mainly as trade languages and were mostly used regionally. Latin served in the ancient times until the period of discovery as a common religious, scholarly and even political / legal language (remnants of this still exist today). But for reasons of politics any language of a particular nation will always be resisted by some groups, leaving the question: Where would we find a neutral language that could serve as a common language?
This has lead to the creation of many artificial, or “constructed languages” (or “ConLangs” as many refer to them). One should note that not all ConLangs were created for universal communication, but many were. But if we, humankind, were to choose a ConLang, which would be the most obvious choice?
Right now the obvious choice of a “universal language” would be Esperanto. Though there are offshoots of Esperanto and many other ConLangs have sprung up since, Esperanto has a massive headstart on almost all of the others! For instance, have you ever heard of Ido, Toki Pona, Logban, Interglossa or even Volapük, the first planned language to achieve any significant degree of success? You can look some of these up on YouTube but have you ever even met anyone who speaks any of these? Me neither. But I speak Esperanto and can find people all over the world who do.