[Formendacil]

I think we have to assume the possibility that there were Edain at the Havens, absent any definitive statement they were NOT: the Edain were proven allies of the Eldar, people who are in terror of their lives flee, and where else were they going to flee to? And we needn't be talking about enough individuals that it would a remarked-upon contingent within the community: it could just be a few escaping refugees fleeing south and taken in.

But there's always the possibility that Eärendil learnt it from the Eldar: even if the Havens were more Doriathrin/Gondolindrim in make-up than aught else, Círdan's community on Balar surely included refugees from the Finarfinian realms: i.e. the realms where "Mannish" probably meant the tongue of Bëor, Barahir, and Beren. Given his sea-faring, it's not unreasonable to think that Eärendil would have had the opportunity to know various members of the Balar community.

Either way, it is likely enough that Eärendil did have to pick up Bëorian as an "extra" language, but that doesn't seem like something to be wondered at in someone with Elvish blood: picking up languages was one of their great skills and delights, and there is definitely evidence somewhere that Eärendil felt a closer kinship with his Mannish side (if I recall correctly, Tolkien states that he chose an Elvish fate despite feeling closer to his father's side): and I don't cite this as proof that he wouldn't be good at learning languages; I'm say that as a half-Elf learning languages, he'd be motivated to learn the tongues of his Mannish side (and either Bëorian refugees or Elves who saw him as more Half-Man in a benign way would be prone, I think, to pass this heritage along).

Certainly, given its kinship to Hadorian proto-Adûnaic, I think Bëorian would have been fairly easy if he had a knowledgeable teacher--a reason, perhaps, why Bëorian would have been learned, rather than the tongue of Haleth--like an Englishman learning Dutch.

It's perhaps even possible to concoct a love story, wherein Eärendil seeking to learn more of his Mannish forebears from the Finrodian element of the mariners on Balar, discovers that he shares not just a kinship but a rare language with Elwing, who learned Bëorian from human refugees that helped her and her nurse to the Havens from the wrack of Doriath.