The threats against Longueuil Mayor Caroline St-Hilaire were posted on social media after she claimed to be "irritated" with a colleague who insisted on speaking both of Canada's official languages at the meetings, saying it was a huge waste of time.
Police have stepped up the mayor's security.
According to the last census, in 2011, French is the mother tongue of seven million Canadians while 19 million responded that they primarily speak English.
"We're in Quebec and city council deliberations must be in French. This is not a bilingual administration, and that's final," St-Hilaire had said on her Facebook page, provoking the angry reactions.
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It comes after two other Quebec political leaders were also targeted in recent years for taking a pro-French stance.
Language tensions in Quebec date back centuries when an English minority sought to impose English as the language of business.
It was also at the center of political upheaval in the 1970s and became a key part of the push for Quebec independence in the decades that followed.