Neither the federal government nor any of the other provinces is required to consider "STOP" a loanword in French, which would allow a stop sign with just "STOP" to be considered to have a message in both French and English. I suspect Québec does so only to blunt the "Stop signs say STOP in France" criticism.
Which is a rather weird criticism when you consider that Quebec is hardly unique in using something other than the English word "STOP" on their signs. In Latin America you will find signs that say "ALTO" or "PARE" (this includes Puerto Rico, by the way - so stop signs aren't even in English everywhere in the US!). In various places throughout the world you can find stop signs with the word in Arabic, Chinese, and all sorts of other languages. Some countries use an image of a hand palm-forward rather than text, or simply a blank octagon.
There's a decent sized gallery here.The consistent use of the English word "STOP" throughout Europe is, globally speaking, anomalous - the result of the Vienna Convention deciding to copy the US stop sign design when they created their standards in 1968.
Of course, this is more detail and nuance than your average traveler is likely to appreciate. Or going to want to appreciate given that the criticism in question, even if objectively silly, fits the desired narrative of criticizing Quebec for wanting to avoid the use of English.