Preamble: I hope I understand you correct: You have multiple documents, each document is written in one language, the content is the same. There is no language mixture inside one document.
I would create a directory per language.
The I separate the document in small files (a file per chapter, perhaps per section. tables and figures in there own files).
Something like this:
Then I would start with a main document (English or the language you know best). Inside the document I add comments as sync point.
After this you need a diff-tool to compare the directories and files. The sync comments inside the document align the texts during comparison.
For the graphics I would create:
graphics contains the language independent graphics. If you have language depending graphics (e.g. with legends) you can add the to the subdirectories.
Then you define:
With a bit more work, you may write a script, that creates a new bi- (or more) lingual document using the parallel.sty. The you can use a printed document version instead the diff tool.