One of my bikes is a French conversion. A 1986 Gitane Victoire. The Vitus tubing is an awesome thing. Yes French bikes are a pain. When I got the frame it only had the fixed BB cup and headset, thank god it at least had that. The adjustable cup is standard ISO. I am running a standard ISO spindle, so no BB lockring for me. I have put about 1000 miles on it thus far and it hasn't been an issue oddly. Quality French bikes, not 80s Puegot, can be worth it. I do worry when it comes time to replace hardware. I will be sourcing rare parts and probably spending a lot of money to do so. At least I know how to wrench...
My other bike is a 1950s Italian track bike. I am a glutton for punishment.