Western Europe used to be much more of a dialect continuum. Every village had their own dialect, and you could understand everyone around you.
But if you went from Castile to Paris, you'd go from hearing Spanish to hearing French. It's just that between them, you had dozens of intermediate languages/dialects that transitioned very smoothly. It's not like today where if you cross a border people go from speaking French to speaking Spanish.
A large part of the nation-building project in Western Europe was to force everyone in the country to learn and use some standard dialect. So very few people now speak Occitan, Picard, Burgundian, etc., and instead speak standard French.
On the one hand, it's a shame to lose all those languages / dialects. On the other hand, the whole purpose of a language is communication, so the fewer distinct languages, the more people are able to speak with and understand each-other.
My guess is that it's a matter of time before everyone on the planet speaks a common language, and the odds are pretty good that language will be English. Which is a bit of a shame, since it's a pretty shitty language in many ways.
Maybe, but Mandarin / Chinese isn't really used outside China. English has about 1.5 billion speakers, and over 1 billion of them speak it as a second language. Chinese has 1.1 billion speakers, but only 200 million speak it as a second language. It seems like the international langua franca is English.
Policies could change, and China could try to teach people Mandarin as part of the Belt and Roads initiative. But, right now, if you're trying to do business in other countries or with foreigners in your country, English is the most useful language to know.
That's not to say that future-English will necessarily be too similar to current English. I'm sure the more it spreads, the more it will change.
But really, isn't the "Franca" part from the Frankish language used in West Germany / Northern France / Benelux? Apparently the Byzantines called all Western Europeans "Franks" and the Lingua Franca was the simplified Italian / Spanish language with many Greek, Slavic, Arabic and Turkish loan words used for trade around the Carribean. So, it's more "English is the European Language" or maybe "English is the language to use with Europeans".
Hopefully with better pronunciation, it was quite grating on my latest rewatch haha
And one episode where the ship's alarm is in Cantonese, for some reason, when they used Mandarin for every other use of Chinese in the whole show ¯_(ツ)_/¯