It has to be remembered that Columbus was a merchant by trade, and by knowing many languages he would have a leg-up in business. Its more probable that Columbus was Genoese considering that he consulted the Elector of Genoa First, because of his national sentiment against Venezia he would have wanted his own government to have a quicker more viable trade to India and China. Unfortunately, Genoa didn't want much to do with his idea, as the small Republic had interests for their ships elsewhere in the Mediterranean. Then Columbus went to Portugal, and asked the King for ships to cross the Atlantic, but Portugal was already headlong in Colonisation, particularly in Africa and India, and they thought it would be an ill investment to pay the fee for several ships to cross an infinite body of water (in their minds). Finally, Columbus was able to strike a deal with Spain, who after finally pushing the Moors out of Iberia, were not in great need of Transport Galleys... or Caravels. Isabella took a gamble with Columbus, if he was successful, to her mind then Spain would have a dominance in Global Monopolies, if unsuccessful, the short term damage was that Spain was out three ships, and quite a bit of money... the long term is left to speculation, but its pretty easy to say, that Spain wouldn't be getting any Boost in Gold Dubloons. But we all know Columbus was correct, and the rest is history...

Going back to my first point, If Columbus wrote his log in Spanish or Catalan, perhaps he did it because he wanted Merchants back in Spain to be able to read it legably without having to know much Italian.