hi, Vincent!
i dont know OSX but with the previous systems you could try to do the following (you have to do this only once
it will remain for all future sessions): apple-menu-controls (where you approach settings for monitors etc.) look for keyboard (clavier)settings. activate the one with the U.S.flag (you can activate as much as you like). i assume that by default you will have the french flag on your apple bar on top of screen. from now on, if you click onto the flag-icon you can access the U.S. keyboard with two clicks only (and switch back to french keyboard as well, of course).
[img]http://www2.zbrushcentral.com/zbc_uploads/user_image-1036043945fqx.jpg[/img]
usually the shiftkey+the one right beside it (in my case its “<”) will do the tilde-ing along x + y axis, holding the alt-key too will do the z-axis. when using the z-axis tilde-ing, sometimes it is switching layers instead, but if you play with this a bit you will get the hang of it. do not forget to switch back to clavier francaise after tilde-ing, as a few shortcuts we got used to wont work. so, the tilde-function is just a click away 
hope this helps
(alternatively check out this thread and info by cameyo - i havent found “126” on the austrian keyboard but you might find it on the french :))