> With a little more macro magic, I would assume it wouldn't be > too much trouble for both pages to have the same content for a > portion of it, such as with an illustration, say, or a math > equation.
Indeed, that should be possible without much work. In the simplest case, one could just duplicate the material, but you might just as well write a macro that simply appends to both diversions. In the example, every language change starts a new paragraph, but this doesn't have to be. The important thing is just that the two diversions are synchronized in length when switching back to the first language (and output if enough material for a whole page has been gathered[*]). [*]This wasn't implemented correctly, but is fixed now. (Everything was just flushed out at the end, which is very inefficient if many pages were being processed, because it results in mostly just copying overflowing formatted material around.) I have now also uploaded a minimal version with a few comments indicating the purpose of the macros, and added a few features, such as the use of separate environments for the two languages (this would be useful for synchronizing the diversions anywhere in mid-sentence without needing a paragraph break, since the partially collected line could remain in the environment, to be resumed later).
