I had a signature certified today (June 2017) by a notary in Montpellier and it was really simple and easy.
I called the notary nearest me and asked if they would certify a signature ("certifier une signature"). They said no problem, that I could stop by anytime. I showed up, signed the document in front of him, and showed my passport as ID. He certified my signature with an official stamp.
The whole thing took 90 seconds and he didn't even charge me.
The document was in English (no French translation), but the notary didn't mind because, as he said, he was only certifying the signature, not the contents of the document.
Thought I would share this here as my experience was so different from everything I had read online about France: going to city hall, needing an official translation, paying $50 at the US embassy, etc.
It was just as straightforward as having something notarized in Canada.
Unrelated: I've also had a document notarized in Indonesia (long story) and it was equally easy.