I typically use multiple dictionaries but not because of language differences...at least not languages found on Earth.
My default User Dictionary is for normal Earth-bound words of any language; the default dictionary is always enabled.
I then have a different User Dictionary based on the series I am editing (eg. "Midkemia", "Pern", "Dune", "Pandemia", or "Witch World"). I add words/Proper Names to that dictionary that are peculiar to that series. I then enable the "world/series" specific dictionary when editing a book to which it applies.
This eliminates almost all of the false spelling errors and leaves the ones that I actually have to look at.
Hope that helps!