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This is a very broad question and could be asked of many names/words in European languages which are all said to be derived from the same Latin word.

How is it the Spanish/English spelling/sound can be so different while both being derived from the same Latin word? I know there's no J in Spanish, but Santiago and James share zero similarities? What am I missing? Please forgive my naivety.

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What you're missing is that the name Santiago is composed by two particles: Sant Iago (Saint + James).

In his Spanish entry in the wikipedia you can find a short explanation of the evolution of the name from the original Jacob:

Su nombre en hebreo es Jacob (יעקב), pero con el tiempo se ha ido deformando en Jacobo, Iago, Yago, San Iago, San Yago, Santiago, Tiago o Thiago, Diego, Jaime, James, Jim, Jimmy, Jackes, Jacques. Decir San Santiago es un error.

How Jacob evolved to James is beyond the scope of this site but according to this other wikipedia entry English takes it from Old French.

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