Boring: they're all found
Better: the find the submarine empty
Even better: only one person left
Best: completely new person found alone on submarine
It’s D B Cooper
Writing a fantasy story where the universal language is something referred to as "common", making it the native language of anywhere (and especially, making it "the human language") is kind of boring and implies imperialistic origins - there's frankly no other way to make almost every character in the setting speak the same language as either their first language or fluent second language than by implying that the native speakers of it invaded land all over the place at some point and forced the locals to learn it, to the point that peoples who never were invaded figured it's easier to just learn it too in order to engage in business with so many other peoples.
I think a more fun alternative would be that "common" formed as some sort of an ultimate pidgin that eventually developed into a more-or-less universal creole. The different peoples that mixed together started using loanwords from each others' languages because their own doesn't have a specific word for that oddly specific thing, and their polyglot kids are already losing tracks which word for "horse" comes from elvish and which one was the original human word for "anvil" or "ship".
While the spreading creole would definitely have local differences, being originally a mashup of dwarvish and gnomish at one end of a continent and a fusion of human tongue and orcish on one end, it would slowly mend together, becoming more standardised over time as the words that are the easiest for each people to pronounce become more common. Eventually the Common Creole becomes so consistent and uniform that almost anyone who speaks one or two of the most common five languages can vaguely pierce together what a conversation spoken in Common is about.
manywinged