[ArchieMaclean]

I haven't tried it, but there is a list of available dictionaries in Plover format [1] including French, Spanish and Italian.

As to switching on the fly, I can't see anything in the docs about it - this extension might work? [2]

[1] https://github.com/openstenoproject/plover/wiki/List-of-Avai...

[2] https://github.com/nsmarkop/plover_system_switcher