Is "BAC+n" understood in Finland?
The ECTS was created precisely for this kind of issue. If you think there is any doubt, just say how many ECTS your degrees are worth. In any case, the Bologna process started about 20 years ago. I think by now everyone knows the lingo. Be aware that the first degree is usually called "bachelor", though. Bac+5 is master's degree, so if you do have a master's degree, just say so.

In any case I would be very surprised if "bac+n" was understood in Finland. You should realize that "bac" refers to "baccalauréat", the French high school diploma. Even assuming that someone understand that "bac+n" means "n years after the bac", they are very likely to think that "bac" is "bachelor", the French "licence"... And "bac+n" is hardly a useful metric anyway, the content of the degree matters much more.