Using classes in different languages is impossible and will never be as far as I know via SO's or DLL's.

You could use things like COM or COBRA but those are winshit only.

The problem lies in the VMT and in the memory layout of classes.
This is not FPC specific it's common for all OOP language(hence COM)

If it's not cruical I'd suggest using classes internaly and exporting only procedures/functions.