The quick answer to your question is, if you buy a recent model you have a very good chance it'll work in France. Now for more detail. 

### Network
France uses predominantly the GSM network while in Canada usage is still mixed, so if you're buying a phone from an operator, make sure you choose a GSM operator such as Rogers or Fido and avoid Bell or Telus which use a different network.

### Frequency

Even if you buy a GSM phone, European frequencies are slightly different so make sure you buy what is called a multiband phone (supporting 850/1900 and 900/1800MHz) [frequencies](https://en.wikipedia.org/wiki/GSM_frequency_bands). You can find these in the phone manual or you can just ask the seller. As for LTE, major Canadian operators [operate](https://en.wikipedia.org/wiki/List_of_LTE_networks#Americas) on the two frequencies (1700 and 2600) used in [Europe](https://en.wikipedia.org/wiki/List_of_LTE_networks_in_Europe) so you should be ok there too.

### Unlock

Only applies if you're buying from an operator, make sure you unlock the phone before returning to France. I've done it many times and is very simple, you pay for a code the operator sends you, install a package and restart your phone.

**Note**

Most recent smartphones, including Apple and Samsung ones, sold in North America support multiple frequencies and you can buy them unlocked if you wish. I've been to France with a Samsung and two different models of iPhones since 2010 and never even had to think about it, they just worked.