Ever since we started buying ABC's cookies (a type of cookies very popular in the USA), the toddler showed interest in learning the letters. We would give him a couple of cookies and tell him: 'Look Monkey, this is a B for boat and a C for car'. And even though he is not even two right now, he happens to recognize quite a few letters. It's clearly too early to go any further, but he loves learning things and showing off in front of friends and family. He is such a character!
It also helped to have a little toy laptop and a toy tablet that Situ (Arabic for grandma) gave him, which teaches the letters, and one of his favorite books: Portland ABC's by John Skewes (a book from the 'Larry gets lost' series).
The problem came when he actually started relating the letter to the word that we told him. Ok, he first learned the letter M (obviously) for monkey/mono and it worked. But, in general, the Spanish and the English words don't start with the same letter. So, if Mama Monkey said: 'Esta es la P, de plátano'. He, afterwards, with his mouth stuffed with cookies, would grab a P and shout: 'Banana!'