French spindles have ISO tapers while Japanese spindles have JIS tapers. The tapers are somewhat compatible because they share the same taper angle, but differ in the starting and ending taper widths. A JIS crank will slide farther than an ISO crank onto an ISO spindle, and conversely an ISO crank will slide less than a JIS crank onto a JIS spindle. Lots of people interchange them, but it can cause problems. For example, seating an ISO crank onto a JIS spindle will result in less contact area along the tapers, but people have raced with this type of configuration and it doesn't lead to catastrophic failure. It may reduce the life of the taper, however.

Sheldon Brown provided a good explanation of this.

I have a set of ISO cranks installed on a JIS spindle, and it works just fine. I estimated that a 115mm JIS spindle would give me the same chainline as a 120mm ISO spindle, and that worked out to be the case.