It's entirely probable they can police the Atlantic (and Gulf) coast (mostly) by the time Columbus shows up, at any rate. Even then, it was still awhile yet until the mainland was found. Even longer until you have to worry about the West Coast or anything, so if most of the efforts are concentrated on keeping settlers out of the East...
(mind you, this all assumes that Columbus even shows up. Butterflies are a thing, and we don't know how wide that particular net is yet)
Insofar as 'trading with Europeans' goes, the English are probably a big one since they aren't really powerful yet, but they share a common language (discounting the fact that English-of-the-15th-Century is going to be missing so many loan words compared to 21st Century English, even without whatever mixing comes in from the downtime Native languages).
...actually, in hindsight, the English that England speaks is going to make little sense to the Americas and vise-versa. Similar enough to freak out England, but otherwise...
Anyway. England is probably a notable option. Maybe France if you want to piss off Spain. I doubt the Spanish will get much trade at first, since they're the biggest colonial types right now. Unless they don't take over Mexico, in which case, their Empire is stillborn without all that juicy gold.