For what concerns to Spanish, as Sonya says it's a little bit difficult to make a difference between Knight and Cavalier, as we use (as you've seen) the word Caballero to both "titles". Also, I think feudal tradition in Spain has been a little bit different from England or France; that would probably be the reason that some nobility ranks haven't got a direct translation in which the meaning is the same.
I think the most accurate word to describe what we understand in game as knight would be Hidalgo in Spanish. This means "Hijo d-algo" (son of something), a low noble rank. These nobles owned some parcel of land that allowed them to live of what this land produced without having to "get their hands dirty" working. Their only duty was to fight for their king when summoned. In Spain, some wealthy enough warriors who could pay for their horse and weapon, gained this rank by fighting against the Moors; the king, in exchange gave them a parcel of the conquered land. A lot of the current common Spanish surnames come from this tradition: Pérez (son of Pero), Díaz (son of Diego), Rodríguez (son of Rodrigo)... This low nobles had the right of being named after their father.
Cavalier is on the other hand a high nobility rank, which I'm not sure if could be translated to Paladín. A brave warrior with high ideals, but humble and pious. Not sure about that, as I've seen this word used in English to talk about knights of Religious Orders.
Anyway, we use Caballero as a generic word for medieval nobles, and literally for knight, cavalier, or even gentleman, regardless of the appropriate meaning or translation of the rank in Spanish.
