As for the polylingual thing, that was a pure assumption.
Considering Wammy Children handle detective cases around the world, they'd need some sort of knowledge or translator to work with various police departments.
Also, Mello, Matt, and Near all lived in England for the majority of their lives, so they'd HAVE to know English, right? Yet once they go to Japan, BOOM, speaking Japanese, considering Near was what, 16 when he went to Japan? I just figured he memorized several languages in order to work all across the world.