Using the style to solve the problem still requires that I select individual Spanish words on each page. Right? I can press and hold Control and double click each Spanish word, then apply a style. But that's still a very tedious process for a few hundred pages. If I misunderstand, kindly let me know.

You did not misunderstand. On the other hand if done as you type, it is not that big a task. As pointed out, if there is already some kind of different formatting applied, a simple Replace will apply the style.

Word simply is not smart enough to figure out what you want formatted one way or the other. When it is allowed to try, it often causes different problems! It will not allow application of two proofing languages to the same word.