I've had several Motobecanes, all were Swiss. It will be marked. None of mine was as old as yours, so hopefully yours will have french thread.
The good news is that a swiss bb is no longer terrible news. IRD sells Swiss cups for its cartridge bb. The cups and bb will run you about $50. In the past, the only option was a Phil Wood model, for about $150.
Last edited by wrk101; 09-05-11 at 06:46 PM.