No solution at all from me, just a suggestion: save the document in rtf and see what happens in the exchange. A rather small test document would be advisable.
You can open an rtf-document with Notepad (shift-right click, Open with ... in Explorer), and then you'll see all the codes.

I don't even know if language settings are saved in the rtf-format (it might be defined single language only), but an experiment might be informative.

The only workaround I can see is that you define your own tags (like [ENG] and [/ENG]) and write some clever macro to select the text in between and set it to the correct language. Take care to write another macro to delete this tags when the document is finalized.

It's an interesting problem. Did you search google for it (try web and groups both)?

Kees