InDesign English Book to be Translated into Spanish

There's a few different questions going on here, but I think that the one to answer first is: yes, we can tell you how to find people who specialize in this sort of thing. But much depends on how much you want to spend on that sort of thing.  My clients are translation firms, and there are web sites like proz.com or translatorscafe.com where I or my clients might go looking for a translator with a particular language pair or a particular specialty. So, if your self-published book is about  e.g. some kind of cool niche interest, then you'd most likely need a translator that knew something about that niche interest.

On the other hand, sometimes my clients come to me with Someone Else's Translations in Someone Else's Canva File, and I can see in the email forwards that someone who's not a language industry insider found someone on, you know, someplace like Fiverr or Upwork or Facebook Marketplace or whatever.  Sometimes, it's even good! (Ya pays ya money and ya takes ya chances.) But that's not a safe way to spend money if you're not already familiar with the ins and outs of the work of translation. You can wind up with something completely unusable, and not find out until you try to share your book with your readership. 

On the technical side,  you'd typically supplyat least an IDML file and a PDF. Translators typically use tools that let then edit markup files like IDML files without any of the actual design software; they don't usually work directly in InDesign. Translations tend to be longer than the source texts; you can bet on your Spanish translation having a word count maybe ten to thirty percent higher than the English. This leads to a fair bit of post-translation cleanup. Unless your document was built very carefully, or modified to accept text expansion gracefully, it's not going to just automagically expand; you (or someone like me!) is going to need to adjust the layout to fit the expanded text. If you're competent InDesign users, it's possible to DIY the post-translation cleanup, but it's much harder to do well if you don't read the target language.

Most of us have, for example, purchased some kind of electronic geegaw from the Internet and found that the manual was laughably bad, right? Written and/or laid out by someone who doesn't know the language at all? So, when someone starts a conversation about translation with me, and the words "self-published book" appear, my first questions are all about money. 

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I realize that this isn't just a simple copy/paste/replace solution, and that the entire flow of the book is going to change.

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By @Dirthmitore

There is not much you can do about that - but either you'll "fix" your document yourself after you get it back - or you can pay extra to the company that will be doing translation to do it for you.

If you are on a tight budget - and you can do translation yourself - texts can be extracted as a WORD document with original text in one column and empty space for translation in another column - and then it can be imported back.

For clarity / ease of use - texts can be exported as separate Paragraph - not as a large chunks.

@Joel Cherney has worked with a translation or two. I believe there's a workflow that will take the IDML export of the book and return it in the same form, so that much of the formatting and layout can be preserved.