I speak Dutch, French, and Mandarin, and I can read and write Latin and Greek. I'm passable in Portuguese but wouldn't claim fluency and would struggle with the Brazilian form of it.
It's more how everything affects magic and how magic affects everything. Words are interesting because they're powerful; there's a society at Yale whose entire specialty is crafting magical language into binding contracts with deadly consequences. What we call "death words" can banish a ghost, but interestingly enough they don't seem bound by language--I've seen Stern banish ghosts that were almost certainly strictly English-speaking with death words in a dialect of Spanish.
The area in which I have the most specialized knowledge, though, is how places affect magic and gather power. There aren't many places in America outside of New Haven where powerful magic is possible, because the land doesn't have the kind of power that New Haven does. I've spent a great deal of study time on Why New Haven and How New Haven and If New Haven.
But I'm going on far too much about myself. What are you interested in, if not magic?