this also happened in the Polish campaign of 1807 when French or German officers conversed with Poles in Latin.

As for the language of liberty = French, in my view a lot of important works about philosophy and literature were published in French in the 18th century which had much more impact than any dubious military schools.
You had to read French - otherwise you would have been out of date and vogue - as indeed you have to read it today - in case your interest are the Napoleonic Wars.