Montreal, the second largest French city in the world after Paris, traditionally has been the stronghold of Quebec's English-speaking population as well.

But under a section of the separatist Parti Quebecois government's language charter that recently went into effect the English profile in the mostly French-speaking province of 6 million people will be much less visible.

The charter, which designates French as Quebec's official language, requires all new commercial signs in the province to be in French only. Existing bilingual signs can stay up until September 1981, and then they, too, must be converted to French.

Cultural Development Minister Camille Laurin, author of the bill, said the legislation is aimed at "changing the face of Quebec."

The changes will be most evident in Montreal. Less than 20 percent of Quebec residents are English speakers, but most of them live in this city of 2.5 million residents and many store fronts reflect their presence.

The switch to French is cheap for some store owners, but costs run into thousands of dollars for others.

Lou Marcotte said he spent "just a few cents" to cover part of his sign over Lou's Hardware. From now on the store will simply be known as "Lou."

But Seven-up Montreal Ltd. will pay $150,000 to make their signs in the area conform to the new regulations.

The soft-drink company's slogan "Fresh-up with 7-Up" has been outlawed, to be replaced on 350 advertising signs with Hop La Vie, which roughly translated means "Life is Great." The fate of the other sales slogan that calls the drink "The UnCola" is uncertain, but if too may be banished, company officials said.

The city of Montreal Permits Department estimated there are about 25,000 commercial signs in the city, but government officials could guess not how many will have to be changed because of the new rules.

English-speaking movie theaters will be able to use English film names on marquees, but posters advertising coming attractions must be bilingual or in French only.

An Odeon Canadian Cinemas official said the film, "Jaws 2" is permitted on the posters but the tagline - "Just when you thought it was safe to go back in the water" - must have a French equilavent or be crossed out.

In addition to changing commercial signs, the rules limit the use of languages other than French in catalogs and leaflets.

A person found guilty of violating the regulations could face a $500 fine for the first offense and $1,000 for the second. Companies can be fined up to $1,000 to $5,000 for the first and second convictions. A court may order a sign not conforming to the law be taken down and destroyed after an eight-day warning period.

But officials who have enforced Quebec's language law since the former Liberal government passed its own softer version three years ago, stress that nobody has ever been charged, much less fined, for violations.

Some companies are not planning to defy the law but they are in no hurry to obey it either.

"I'm just going to wait until the government sends me a letter telling me to change my sign," said the owner of a grocery store in the city's mainly English-speaking West End.

Royal Bank of Canada argued that, as a federally incorporated company, the government could not order it to change to French. It said it would continue to use signs in its Quebec branches written in the language used by most customers in the area.