There is a forum that discusses the Ontario Highway Traffic Act, and this is apparently a question that gets asked a lot. To sum up the discussion, because of O.Reg 615 of the French Language Services Act, there is a possibility that you can challenge uniligual signs in French service regions like Toronto. The reality is, Toronto never adopted the option to make all traffic signs bilingual. Quoting someone who posted in the forum who tried to challenge a sign without french and lost the case:
"...the prosecutor argued that City of Toronto never approval or adopted
this bilingual option. He further mentioned that the City has appealed
and won the case from the superior court."
From the Highway Traffic Act:
HTA OREG 615
52. A municipality situated in an area designated by the French Language Services Act is not required to comply with the sign
requirements for such areas unless it (Municipality) has passed a
by-law under section 14 of that Act.
So the answer appears to be yes, you have to obey English-only signs in Toronto. You may be able to challenge signs in other areas of Ontario, but only if a municipality has passed a by-law under section 14 of the French Language Services Act.
Yes, you have to obey all posted signs. Depending on the region, you could make an argument that the sign must be updated to include french, but it is unlikely you could argue that you were not required to obey it in most places.
Canada is bilingual at the federal level, so all federal services must make their signs bilingual, but the provinces of Canada are not all bilingual. The laws will change from province to province, Quebec for example is technically unilingual French, so they are not required to put english on their signs, but regardless of which province you are in, you must obey all posted signs.
There are a number of provinces that are unilingual English, I grew up in British Columbia, there aren't any French or bilingual signs out here except for in National Parks and on government buildings. I couldn't carry a conversation in French if my life depended on it, not many people out here could, and the ones that can mostly moved here from Quebec.
Ontario has the French Language Services Act, which requires all government services (not just federal) to be offered in french if the francophone population meets a certain threshold. The areas in blue in the image below represent areas that provide french services, the dark blue indicates that the entire region is required to offer french services, the light blue indicate areas that have communities that offer french services.
Toronto is a designated french services municipality,
so technically, the sign probably should be bilingual, but the fact that it is not does not give you justification to break the law. Like I already said, you could file a complaint and have the city update the sign, but you're not likely to get away with a traffic violation based on the fact that the sign wasn't bilingual.