English is without a doubt the prefered language of communication among people who don't share a native tongue. But when did English gain this status? I'm thinking it was probably after WW2, when the US became a superpower. While it might happened earlier (maybe after the US stepped into the world stage after WW1), in my country English didn't become the prefered foreign language for school students until the 1980s, so I believe it wasn't too early.
And about the second part of my question: barring languages that were used regionally (like German in central Europe or Russian in eastern Europe), what language was the lingua franca before English? My hypothesis is French, which had been for centuries the prefered language of diplomacy and was highly regarded as such well into the 20th century. In fact, in my country it was French that was replaced by English as first foreign language at schools. However, I only know French was used in diplomacy - what about the common people who travelled abroad? Tourists, traders, scientists, students... What language did they use to communicate with foreign people before English became so important?
English began to make its ascent during World War I (the Treaty of Versailles was written in both English and French). The expansiveness of the British Empire helped English become a dominant language, but you're right -- it really took off after World War II, thanks in large part to American Culture/Hollywood. The internet was also a big driver, about 50% of the internet is published in English.
And yes, Lingua Franca was originally French. There wasn't really a lot of common people that travelled abroad up until the second half of the 20th century. Free time wasn't really a thing for middle class people. But it really depended. If you look at books like Goodbye to Berlin, (which is semi-autobiographical) the narrator, an Englishman, talks about his inability to grasp the German language, so he gets along by speaking broken German/hoping people speak some English. Essentially, unless you were carrying out official business, French was the preferred second language that people should know, unless you lived in a colony, and then it would be the third (your mother tongue, the language of the colonists being the first two).