it's because in the maybe mid 70s, or 80s, there became a hugeee dispute between the french and the english, and it's been a pretty big messy thing, to a point where there were riots and fights, violence etc. basically the seperatists, and political groups and french activists wanted quebec to be a french only speaking province. so even to date, all of the laws about where you go to school are based on where your parents went to school, i.e. if my parents when to english school i have either the right to go to french school, or if one of my parents went to french school i can choose. anyways, it's pretty fucked up. even in college you are required by law to attend 2 mandatory french classes if you attend an english college.
but to the point, after all this bullshit about language and who's right and who's wrong, there were a bunch of laws made up about everything including signs, and store and company names, and etc. if you open a shop in quebec, if it's brand new, and has no ties to anywhere else, the name has to include a french word, and all english writing on signs has to be a specific percentage smaller than the french, and the english always goes below the french. it's all pretty ridiculous. so at one point we had signs that read stop/arrete but people started painting over the stop, and then eventually the stop was abolished all together.
i was just looking at a website that showed a few other countries that have their signs in their language, like places in china, but most people use stop, and on occasion their native or commonly spoken languages in the area.