Why not work in Layers and have a layer for English text and a layer for Spanish text, French text, etc.?
Others may choose to have Conditional Text turned on and therefore make an English condition, a Spanish condition, a French condition, and so on.
Layers seems okay, but I'm not sure to work practical on this in fact the book has 400 pages?
So if I unterstand you correctly, you meant that I should create one layer for every language and within e.g. Layer EN I will create every english textfield and so on.
I'm just not sure how to handle the different size/length of the different languages within the same document, to don't destroy formatting.
For example width of tablecolums: FR (language) wording is longer than EN wording.
For one language I need one extra line at the end of one passage..
What you are describing is very typical of the design challenges faced by those who have to layout multiple languages.
If this is your first time designing this way, you need to get multiple language copies of other books and see how they were laid out.
Perhaps the company that has hired you for this project has some examples.