How to translate an InDesign layout to Spanish efficiently

Job it out. Produce your English file, package it, and hand it off to a professional. 

There are many tools you could use to handle the production of a Spanish version of your English layout. The simplest and most straightforward of these tools are plugins it scripts that rely on hoovering up your text content, running it through AI translation, and automatically inserting the translated content. At that point you get to reengineer the document, as you can expect your Spanish translation to be around 15% to 20% longer than your English was, so your text objects are likely to shift or overflow. 

Now you have a Spanish document that no living Spanish reader has ever looked at, so you now need a proofreader, lest you ship your map and guide to your client with obvious AI hallucinatory content in the middle of your Spanish translation. I know plenty of translators who offer PMTE ("post machine translation editing") but you might not. I always find it hard to hire new editors when I don't already have a trusted in-language editor or two to ask "hey, these edits are from a new kid...  Are they any good?

It's simplest for you to job it out. If the contract  language forbids subcontracting, well, then, I would not suggest going it alone. 

"But I want to go it alone!" Take a look at id-extras or idmls.ai or Redokun, and may God have mercy on your soul.