The same clinic, two languages, two rankings
Search "clinique dentaire" in Montreal with the interface set to French, then run "dental clinic" with it set to English. Same city. Different pack. A clinic that owns the French results can sit three rows down in English, and it happens the other way just as often. That gap is hl, and most Canadian SEO audits never open it.
hl is the host-language parameter. It sets Google's interface language, and it quietly biases which language of pages and snippets Google decides to show you. hl=en and hl=fr are not two skins on one result. In a bilingual market they're two different result sets.
What hl actually does
Be precise here, because people overstate it. hl does not relocate you; that's gl and uule. It does not hard-filter by language the way the old lr parameter did. What it does is set the interface and lean Google toward results in that language, most visibly when the query is ambiguous or bilingual.
So on a French query from a French interface, French Business Profiles, French pages, and French snippets get a leg up. Flip to English and the weighting shifts under you. Same businesses in the database. Different ones surfaced.
To test a Canadian market properly, pin the country and move only the language:
https://www.google.ca/search?q=dentiste&gl=ca&hl=fr
https://www.google.ca/search?q=dentist&gl=ca&hl=en
Keep gl=ca fixed. Change hl. Now language is the only variable, and whatever moves is language doing the moving.
Quebec is where this bites
Outside Quebec you can often survive on English-only checks. Inside it, that's negligence.
Two reasons, and they compound. First, people type differently. Quebec French isn't textbook Parisian French, and the words locals actually search diverge, so "dépanneur" opens a world that "convenience store" never touches. Second, the law now pushes French to the front. Under Bill 96, a business with a genuine Quebec presence is expected to run a French Google Business Profile with French given at least equal prominence, not one bilingual listing mumbling in both languages. Website fines climb into six figures. So the French side of the SERP is getting denser and better-optimized on purpose, which means the English-only view you've been trusting drifts further from what a Montreal customer sees every day.
What moves when you flip hl
Run the two side by side and the differences pile up fast.
The businesses move first. A competitor with a polished French profile can outrank your bilingual-but-lazy one on hl=fr and vanish on hl=en. If you only ever checked English, you never met them, and you can't beat a rival you can't see.
Then the copy itself changes. Under hl=fr Google pulls the French title and description of a page, so a listing that reads sharp in English can land clumsy in French when nobody bothered to write the French version.
Two quieter shifts ride underneath. Intent: "avocat" is both a lawyer and a fruit, and the language you set is Google's only clue which one you meant. And the ads on top of the pack often swap out completely, because French and English campaigns get built and budgeted as separate things.
How I'd run it
Pick a bilingual market: Montreal, Ottawa, Moncton, stretches of northern Ontario. Take one query and write its honest French and English forms, not a Google Translate pair. Fix gl=ca, fix the location, flip hl. Read both packs.
The businesses that show up in one and not the other are your blind spot. In Quebec that blind spot is almost always the French one, because that's the side most anglophone agencies quietly skip. Whether you hand-build the two URLs or flip the language in a tool that does it for you, the point stands: if you've never looked at the French pack, you don't actually know your own market.