The ones I saw who were able to do it learned French very quickly. We're really talking about complete immersion both culturally and linguistically, and they don't lose their French. I remember that.
Unfortunately, it is not always easy to arrange. Often it is because of the family. People have families, which is perfectly legitimate. As a result, there is always a little bit of tension around that. Even if the employee says he wants to go to Montreal for six months, that it will be fun and he'll be able to work rather than sitting in a classroom, that it will be productive and he'll be learning, it's not always very easy to arrange.