"Marcus speaks German in the nearest library."
Translation:Marcus in bibliotheca proxima Germanice loquitur.
I am not positive in Latin, but in Italian prossimo/a (from proxima) means "next" or "the next". The word vicino/a in Italian means "nearby". For example, la prossima fermata means "the next stop" when you are on a train or bus, but la fermata dell'autobus è vicina means "the bus stop is near/nearby". So, based on this alone my GUESS would be it refers specifically to "the closest/nearest" library. However, I have already discovered many "false friends" between Latin and Italian, so I might be wrong in my assumption.