Can I use a French keyboard on a refurbished English iMac?

You can set the input language in macOS system settings to anything you want. Including French. The keys will then produce the French letters that are in that place.

You can also connect a keyboard with French lettering to the Mac directly and again set the input language to French.

ONLY buy a refurbished Mac directly from Apple!! PLEASE NEVER EVER purchase what is purported (it's a lie) refurbished Mac from:

• Ebay
• Craigslist
• Nextdoor
• Amazon
• Marketplace
• Local PC Store
• Friend, Neighbor, Relative and of course a stranger

Why, very very simple. First, in most cases these are not really refurbished, they are simply used machine that have installed low quality replacement parts. They are installed by non-Apple trained techs, very frequently they have not be setup correctly for resale (meaning they will NEVER run correctly). Only purchased an Apple refurbished machine directly from Apple, navigate to Apple.com, scroll to the page bottom and select the Refurbished Store. These machines are identical to new, have a new warranty (same as a new machine) are eligible for AppleCare and can save hundreds of dollars. These are also eligible for Apple's 12 month no interest financing.

If you still cannot afford and Apple Refurbished buy from a reputable reseller, in North America (at least Trump hasn't changed that name) that would be macsales.com.

Back to your question though, just wander into an Apple Store or reseller in Quebec and buy a French language keyboard.

All the accents needed for French are easily made on an English machine via press and hold on the keys or via option key shortcuts.

Enter characters with accent marks on Mac - Apple Support

You're welcome.