Years ago, I made a training program in different languages, each language in a separate document. After a year, I get several corrections: new courses, delete courses, changes in prices. It was a mess, to change everything in those independent documents.
Now I have everything in one document on different layers as you described. German is the master language. If this layer is finnished, I make the changes in other languages. Because everything is in one document, I must insert or delete pages only once.
Every language has it’s own set of para and char styles. Textframes are sometimes named. Because of this naming-thing, I can control the flow of text and also the visibility of layers with scripts. E.g. if you have multiple layers, you can skip the Export-Dialog and do everything with a script > make 3 layers visible, export to PDF, set the visibilty after the export to false …
To your layout question: Assume you built your document from scratch. It should be possible to work either with named frames or duplicate the finnished layer (everything with scripting), if you do not want to layout with stacking frames.
So with a bit of brainstorming before the project, many things can be improved.
Kai