I don't recommend putting anything on the back of a business card because it's a place where people write notes. Say for example, someone asks you for a price to do something, or the name of a company that does XYZ. You write it on the back your business card and hand it to them. They now have a reason to keep your card.
Now, if your business card was black on the back or is full of pre-printed information, your clients won't be able to write notes… assuming that they even know what to do with a business card (most people don't). If you want to put stuff on the back, either organize it so that there is still room to write a few notes or use a folding card which is more expensive but would meet your need.
Also, if you have a bilingual business, you might consider putting the English and French versions each on their own side of the card.