GOT IT! GOT IT!
My mom and I put our heads together over Poirot's French message, and what we think he's saying (with... less-than-stellar grammar, though I suppose Canadian French is different from the Belgian dialect) is: "Before I go, the first thing to do is you have to know that you're able to do it."
...I know, a little late, especially if that was an encouraging message for us/Adell about fighting Cthulhu.
But hey, better late than never, right? And there's a trope for that... trying to remember what it is, so we can give it to Poirot.
And NOW I'm gone for a while.