Not to get you worried here, but it is possible to screw an English/ISO freewheel onto a French hub. It will be a loose fit and will screw on most of the way before showing resistance due the very slightly finer French threads (1.000mm vs 1.058mm). Everything will seem fine until you put some hard effort into the cranks, and then strip out the hub threads. I'd only feel confident if you tried a known French threaded freewheel, and it would not thread onto the hub by hand. Remember, there is only a 0.1mm difference in diameter between English and French threads, and 0.22mm between ISO and French.